1.0 Chapter Introduction
1.0.1 Chapter IntroductionPage 1:
Today's networks have a significant impact on our lives - changing the way we live, work, and play. Computer networks - and in a larger context the Internet - allow people to communicate, collaborate, and interact in ways they never did before. We use the network in a variety of ways, including web applications, IP telephony, video conferencing, interactive gaming, electronic commerce, education, and more.
At the center of the network is the router. Stated simply, a router connects one network to another network. Therefore, the router is responsible for the delivery of packets across different networks. The destination of the IP packet might be a web server in another country or an e-mail server on the local area network. It is the responsibility of the routers to deliver those packets in a timely manner. The effectiveness of internetwork communications depends, to a large degree, on the ability of routers to forward packets in the most efficient way possible.
Routers are now being added to satellites in space. These routers will have the ability to route IP traffic between satellites in space in much the same way that packets are moved on Earth, thereby reducing delays and offering greater networking flexibility.
In addition to packet forwarding, a router provides other services as well. To meet the demands on today's networks, routers are also used to:
- Ensure 24x7 (24 hours a day, 7 days a week) availability. To help guarantee network reachability, routers use alternate paths in case the primary path fails.
- Provide integrated services of data, video, and voice over wired and wireless networks. Routers use Quality of service (QoS) prioritization of IP packets to ensure that real-time traffic, such as voice, video and critical data are not dropped or delayed.
- Mitigate the impact of worms, viruses, and other attacks on the network by permitting or denying the forwarding of packets.
1.0.1 - Chapter Introduction
The diagram depicts the front panel of a Cisco 1841 Integrated Services Router (ISR). In this chapter, you learn to:
- Identify a router as a computer with an operating system (OS) and hardware designed for the routing process.
- Demonstrate the ability to configure devices and apply addresses.
- Describe the structure of a routing table.
- Describe how a router determines a path and switches packets.
The diagram depicts the front panel of a Cisco 1841 Integrated Services Router (ISR). In this chapter, you learn to:
- Identify a router as a computer with an operating system (OS) and hardware designed for the routing process.
- Demonstrate the ability to configure devices and apply addresses.
- Describe the structure of a routing table.
- Describe how a router determines a path and switches packets.
1.1 Inside the Router
1.1.1 Routers are ComputersPage 1:
Routers are Computers
A router is a computer, just like any other computer including a PC. The very first router, used for the Advanced Research Projects Agency Network (ARPANET), was the Interface Message Processor (IMP). The IMP was a Honeywell 316 minicomputer; this computer brought the ARPANET to life on August 30, 1969.
Note: The ARPANET was developed by Advanced Research Projects Agency (ARPA) of the United States Department of Defense. The ARPANET was the world's first operational packet switching network and the predecessor of today's Internet.
Routers have many of the same hardware and software components that are found in other computers including:
- CPU
- RAM
- ROM
- Operating System
1.1.1 - Routers Are Computers
The animation depicts a simple network topology with router R1 highlighted.
Network Topology:
PC1 is connected to switch S1, which is connected to router R1. Router R1 is connected to router R2 via a WAN link. PC2 is connected to R2.
Initially, the animation depicts a front view of an 1841 ISR. Next the top cover is removed from the router, showing its internal system components. Then a sample output from the show i p route command is shown.
Output from the show i p route command reveals that network 192.168.1.0/24 is directly connected to the R1 LAN interface, FastEthernet0/0. The output also shows that network 192.168.2.0/24 is directly connected to the R1 WAN interface, Serial0/0,
R1#show i p route
Codes:
C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
1 - I S-I S, L1 - I S-I S level-1, L2 - I S-I S level-2, ia - I S-I S inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
C192.168.1.0/24 is directly connected, FastEthernet0/0
C192.168.2.0/24 is directly connected, Serial0/0
The animation depicts a simple network topology with router R1 highlighted.
Network Topology:
PC1 is connected to switch S1, which is connected to router R1. Router R1 is connected to router R2 via a WAN link. PC2 is connected to R2.
Initially, the animation depicts a front view of an 1841 ISR. Next the top cover is removed from the router, showing its internal system components. Then a sample output from the show i p route command is shown.
Output from the show i p route command reveals that network 192.168.1.0/24 is directly connected to the R1 LAN interface, FastEthernet0/0. The output also shows that network 192.168.2.0/24 is directly connected to the R1 WAN interface, Serial0/0,
R1#show i p route
Codes:
C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
1 - I S-I S, L1 - I S-I S level-1, L2 - I S-I S level-2, ia - I S-I S inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
C192.168.1.0/24 is directly connected, FastEthernet0/0
C192.168.2.0/24 is directly connected, Serial0/0
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Routers are at the network center
Typical users may be unaware of the presence of numerous routers in their own network or in the Internet. Users expect to be able to access web pages, send e-mails, and download music - whether the server they are accessing is on their own network or on another network half-way around the world. However, networking professionals know it is the router that is responsible for forwarding packets from network-to-network, from the original source to the final destination.
A router connects multiple networks. This means that it has multiple interfaces that each belong to a different IP network. When a router receives an IP packet on one interface, it determines which interface to use to forward the packet onto its destination. The interface that the router uses to forward the packet may be the network of the final destination of the packet (the network with the destination IP address of this packet), or it may be a network connected to another router that is used to reach the destination network.
Each network that a router connects to typically requires a separate interface. These interfaces are used to connect a combination of both Local Area Networks (LANs) and Wide Area Networks (WANs). LANs are commonly Ethernet networks that contain devices such as PCs, printers, and servers. WANs are used to connect networks over a large geographical area. For example, a WAN connection is commonly used to connect a LAN to the Internet Service Provider (ISP) network.
In the figure, we see that routers R1 and R2 are responsible for receiving the packet on one network and forwarding the packet out another network toward the destination network.
1.1.1 - Routers Are Computers
The animation depicts a simple network topology and packet movement to demonstrate that routers are responsible for directing packets on an internetwork to their proper destination. The animation also shows that routers use different media (cabling) and interfaces for LAN and WAN connections.
Network Topology:
Three PC's and an IP phone are connected to switch S1, which is connected to router R1. Router R1 is connected to router R2 via a WAN link. Three PC's and an IP phone are connected to switch S2, which is connected to router R2.
As the animation progresses, a packet from PC1 on LAN A travels to switch S1 and then travels to router R1. From R1, the packet travels across the WAN to R2. From R2, the packet travels across LAN B to switch S2 until it reaches its destination of PC2.
The animation depicts a simple network topology and packet movement to demonstrate that routers are responsible for directing packets on an internetwork to their proper destination. The animation also shows that routers use different media (cabling) and interfaces for LAN and WAN connections.
Network Topology:
Three PC's and an IP phone are connected to switch S1, which is connected to router R1. Router R1 is connected to router R2 via a WAN link. Three PC's and an IP phone are connected to switch S2, which is connected to router R2.
As the animation progresses, a packet from PC1 on LAN A travels to switch S1 and then travels to router R1. From R1, the packet travels across the WAN to R2. From R2, the packet travels across LAN B to switch S2 until it reaches its destination of PC2.
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Routers determine the best path
The primary responsibility of a router is to direct packets destined for local and remote networks by:
- Determining the best path to send packets
- Forwarding packets toward their destination
It is very likely that a router will receive a packet that is encapsulated in one type of data link frame, such as an Ethernet frame and when forwarding the packet, the router will encapsulate it in a different type of data link frame, such as Point-to-Point Protocol (PPP). The data link encapsulation depends on the type of interface on the router and the type of medium it connects to. The different data link technologies that a router connects to can include LAN technologies, such as Ethernet, and WAN serial connections, such as T1 connection using PPP, Frame Relay, and Asynchronous Transfer Mode (ATM).
In the figure, we can follow a packet from the source PC to the destination PC. Notice that it is the responsibility of the router to find the destination network in its routing table and forward the packet on toward its destination. In this example, router R1 receives the packet encapsulated in an Ethernet frame. After decapsulating the packet, R1 uses the destination IP address of the packet to search its routing table for a matching network address. After a destination network address is found in the routing table, R1 encapsulates the packet inside a PPP frame and forwards the packet to R2. A similar process is performed by R2.
Static routes and dynamic routing protocols are used by routers to learn about remote networks and build their routing tables. These routes and protocols are the primary focus of the course and will be discussed in detail in later chapters along with the process that routers use in searching their routing tables and forwarding the packets.
Links
"How Routers Work" http://computer.howstuffworks.com/router.htm
1.1.1 - Routers Are Computers
The animation depicts a network topology and shows the routing tables of R1 and R2 as the packet moves through the network.
Network Topology:
The network topology from the previous diagram is displayed, but with network IP addresses identified. LAN A, with a network address 192.168.1.0/24, is connected to switch S1. S1 is connected to router R1. R1 is connected to router R2 via a WAN link with a network address of 192.168.2.0/24. R2 is connected to switch S2 on LAN B, with a network address of 192.168.3.0/24.
As the animation progresses, a packet from PC1 on LAN A travels to switch S1 and then to router R1. When the packet arrives at R1, R1's routing table is displayed. R1 looks up the destination network to determine the interface on which to forward the packet. R1 forwards the packet to interface Serial0/0/0. The packet travels across a WAN link to R2. When the packet arrives at R2, R2's routing table is displayed. R2 looks up the destination network to determine the interface on which to forward the packet. R2 forwards the packet to interface FastEthernet0/0. The packet travels across LAN B to switch S2 until it reaches its destination of PC2. Routers use the routing table like a map to discover the best path for a given network.
The animation depicts a network topology and shows the routing tables of R1 and R2 as the packet moves through the network.
Network Topology:
The network topology from the previous diagram is displayed, but with network IP addresses identified. LAN A, with a network address 192.168.1.0/24, is connected to switch S1. S1 is connected to router R1. R1 is connected to router R2 via a WAN link with a network address of 192.168.2.0/24. R2 is connected to switch S2 on LAN B, with a network address of 192.168.3.0/24.
As the animation progresses, a packet from PC1 on LAN A travels to switch S1 and then to router R1. When the packet arrives at R1, R1's routing table is displayed. R1 looks up the destination network to determine the interface on which to forward the packet. R1 forwards the packet to interface Serial0/0/0. The packet travels across a WAN link to R2. When the packet arrives at R2, R2's routing table is displayed. R2 looks up the destination network to determine the interface on which to forward the packet. R2 forwards the packet to interface FastEthernet0/0. The packet travels across LAN B to switch S2 until it reaches its destination of PC2. Routers use the routing table like a map to discover the best path for a given network.
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1.1.1 - Routers Are Computers
The diagram depicts photographs of the 1841 ISR with front and rear views.
Front view:
The 1841 is a relatively low-cost ISR designed for small to medium-sized businesses and small enterprise branch offices. It combines the features of data, security, and wireless services.
Light emitting diodes (L E D's) indicate the connection status of each port. The following highlighted L E D's are described:
System Power L E D - Indicates the presence of power to the device; Solid-green L E D.
System Activity L E D - Blinking when any packets are transmitted or received on any WAN or LAN, or when monitoring system activity.
Rear View:
The 1841 ISR uses modules that allow for different configurations of ports.
The following highlighted components are described:
- 4-port Cisco EtherSwitch 10BASE-T/100BASE-TX auto-sensing
- H WIC (high-speed WAN Interface Card)
- Compact flash module
- Single slot USB port
- Fast Ethernet port 0/1
- Fast Ethernet port 0/0
- Console Port
- Auxiliary port
- High-speed WAN interface card (H WIC) slots
The diagram depicts photographs of the 1841 ISR with front and rear views.
Front view:
The 1841 is a relatively low-cost ISR designed for small to medium-sized businesses and small enterprise branch offices. It combines the features of data, security, and wireless services.
Light emitting diodes (L E D's) indicate the connection status of each port. The following highlighted L E D's are described:
System Power L E D - Indicates the presence of power to the device; Solid-green L E D.
System Activity L E D - Blinking when any packets are transmitted or received on any WAN or LAN, or when monitoring system activity.
Rear View:
The 1841 ISR uses modules that allow for different configurations of ports.
The following highlighted components are described:
- 4-port Cisco EtherSwitch 10BASE-T/100BASE-TX auto-sensing
- H WIC (high-speed WAN Interface Card)
- Compact flash module
- Single slot USB port
- Fast Ethernet port 0/1
- Fast Ethernet port 0/0
- Console Port
- Auxiliary port
- High-speed WAN interface card (H WIC) slots
Page 5:
This Packet Tracer Activity shows a complex network of routers with many different technologies. Be sure to view the activity in Simulation Mode so that you can see the traffic traveling from multiple sources to multiple destinations over various types of media. Please be patient as this complex topology may take some time to load.
Click the Packet Tracer icon for more details.
1.1.1 - Routers Are Computers
Link to Packet Tracer Exploration:
Corporate Network Simulation
This Packet Tracer Activity shows a complex network of routers with many different technologies. Be sure to view the activity in Simulation Mode so that you can see the traffic travelling from multiple sources to multiple destinations over various types of media. Please be patient as this complex topology may take some time to load.
Link to Packet Tracer Exploration:
Corporate Network Simulation
This Packet Tracer Activity shows a complex network of routers with many different technologies. Be sure to view the activity in Simulation Mode so that you can see the traffic travelling from multiple sources to multiple destinations over various types of media. Please be patient as this complex topology may take some time to load.
1.1.2 Router CPU and Memory
Page 1:
Although there are several different types and models of routers, every router has the same general hardware components. Depending on the model, those components are located in different places inside the router. The figure shows the inside of an 1841 router. To see the internal router components, you must unscrew the metal cover and take it off the router. Usually you do not need to open the router unless you are upgrading memory.
1.1.2 - Router CPU and Memory
The diagram depicts inside and back views of the 1841 ISR router.
The components highlighted for the inside router view:
- Power Supply
- Shield for WAN interface card WIC or high-speed WIC (H WIC)
- Shield for WAN interface card WIC or high-speed WIC (H WIC)
- Fan
- Synchronous dynamic RAM (S D RAM) used for holding the running configuration and routing tables, and for supporting packet buffering
- Nonvolatile RAM (NV RAM) and boot flash memory used for storing the ROMMON boot code as well as NV RAM data
- CPU
- Advanced Integration Module (AIM) option that offloads processor- intensive functions such as encryption from the main CPU
The components highlighted for the back router view:
- High-speed WIC (H WIC)
- Flash memory used for storing the software image, configuration files, and log files. Flash memory for the 1841 is implemented in an external Compact Flash memory card.
- USB port
- Fast Ethernet ports
- High-speed WIC (H WIC)
- Power cable connector
The diagram depicts inside and back views of the 1841 ISR router.
The components highlighted for the inside router view:
- Power Supply
- Shield for WAN interface card WIC or high-speed WIC (H WIC)
- Shield for WAN interface card WIC or high-speed WIC (H WIC)
- Fan
- Synchronous dynamic RAM (S D RAM) used for holding the running configuration and routing tables, and for supporting packet buffering
- Nonvolatile RAM (NV RAM) and boot flash memory used for storing the ROMMON boot code as well as NV RAM data
- CPU
- Advanced Integration Module (AIM) option that offloads processor- intensive functions such as encryption from the main CPU
The components highlighted for the back router view:
- High-speed WIC (H WIC)
- Flash memory used for storing the software image, configuration files, and log files. Flash memory for the 1841 is implemented in an external Compact Flash memory card.
- USB port
- Fast Ethernet ports
- High-speed WIC (H WIC)
- Power cable connector
Page 2:
Router Components and their Functions
Like a PC, a router also includes:
- Central Processing Unit (CPU)
- Random-Access Memory (RAM)
- Read-Only Memory (ROM)
CPU
The CPU executes operating system instructions, such as system initialization, routing functions, and switching functions.
RAM
RAM stores the instructions and data needed to be executed by the CPU. RAM is used to store these components:
- Operating System: The Cisco IOS (Internetwork Operating System) is copied into RAM during bootup.
- Running Configuration File: This is the configuration file that stores the configuration commands that the router IOS is currently using. With few exceptions, all commands configured on the router are stored in the running configuration file, known as running-config.
- IP Routing Table: This file stores information about directly connected and remote networks. It is used to determine the best path to forward the packet.
- ARP Cache: This cache contains the IPv4 address to MAC address mappings, similar to the ARP cache on a PC. The ARP cache is used on routers that have LAN interfaces such as Ethernet interfaces.
- Packet Buffer: Packets are temporarily stored in a buffer when received on an interface or before they exit an interface.
ROM
ROM is a form of permanent storage. Cisco devices use ROM to store:
- The bootstrap instructions
- Basic diagnostic software
- Scaled-down version of IOS
Flash Memory
Flash memory is nonvolatile computer memory that can be electrically stored and erased. Flash is used as permanent storage for the operating system, Cisco IOS. In most models of Cisco routers, the IOS is permanently stored in flash memory and copied into RAM during the bootup process, where it is then executed by the CPU. Some older models of Cisco routers run the IOS directly from flash. Flash consists of SIMMs or PCMCIA cards, which can be upgraded to increase the amount of flash memory.
Flash memory does not lose its contents when the router loses power or is restarted.
NVRAM
NVRAM (Nonvolatile RAM) does not lose its information when power is turned off. This is in contrast to the most common forms of RAM, such as DRAM, that requires continual power to maintain its information. NVRAM is used by the Cisco IOS as permanent storage for the startup configuration file (startup-config). All configuration changes are stored in the running-config file in RAM, and with few exceptions, are implemented immediately by the IOS. To save those changes in case the router is restarted or loses power, the running-config must be copied to NVRAM, where it is stored as the startup-config file. NVRAM retains its contents even when the router reloads or is powered off.
ROM, RAM, NVRAM, and flash are discussed in the following section which introduces the IOS and the bootup process. They are also discussed in more detail in a later course relative to managing the IOS.
It is more important for a networking professional to understand the function of the main internal components of a router than the exact location of those components inside a specific router. The internal physical architecture will differ from model to model.
Links
View the "Cisco 1800 Series Portfolio Multimedia Demo," http://www.cisco.com/cdc_content_elements/flash/isr_demo/demo.htm
1.1.2 - Router CPU and Memory
The diagram depicts a logical block diagram of the 1841 ISR internal components.
The following components are not connected to a bus:
- Aux port
- Console port
The following components are connected to the CPU bus:
- User Interface Dual U ART
- Compact Flash Memory Card
- Flash 32, 64 or 128 MB, default is 32 MB
- Boot ROM, NV RAM, 2 or 4 MB Flash Memory
- CPU M860 Processor
- System Control ASIC
The following components are connected to the system bus:
- System Control ASIC
- S D RAM DIMM's 128 MB (expandable to 348 MB)
- Slot 0 H WIC/WIC/V WIC
- Slot 1 H WIC/WIC/V WIC
- FastEthernet 0/0
- FastEthernet 0/1
Descriptions are provided for various components and combinations of components.
Aux port: The AUX port is a management port used for remote configuration of the device. It is most often used for the connection of a modem to manage the device when the network path has failed. Not all routers have an AUX port.
Console port: The console port allows for local configuration of the device. It is a management port and is not designed as a networking port.
User Interface DUAL U ART: Universal Asynchronous Receiver/Transmitter (U ART); two access methods through the Auxiliary and Console ports.
Compact Flash and Flash: Stores the Cisco I O S software image on either the Flash single in-line memory module (SIMM), or the PCMCIA card.
Boot ROM: Holds bootstrap, ROM monitor, and optionally scaled-down version of I O S software; shares flash memory with NV RAM.
NV RAM: Holds startup configuration; shares flash memory with Boot ROM.
CPU: Central processing unit is the brain of the computer. The CPU is where most calculations take place by interpreting the computer program instructions and processing data.
System Control ASIC: Controls the flow of data between the memory, interface, and the CPU.
S D RAM DIMM's: Hold running configuration, routing table, and other dynamic structures.
Interfaces: Two modular slots and two built-in Fast Ethernet ports.
The diagram depicts a logical block diagram of the 1841 ISR internal components.
The following components are not connected to a bus:
- Aux port
- Console port
The following components are connected to the CPU bus:
- User Interface Dual U ART
- Compact Flash Memory Card
- Flash 32, 64 or 128 MB, default is 32 MB
- Boot ROM, NV RAM, 2 or 4 MB Flash Memory
- CPU M860 Processor
- System Control ASIC
The following components are connected to the system bus:
- System Control ASIC
- S D RAM DIMM's 128 MB (expandable to 348 MB)
- Slot 0 H WIC/WIC/V WIC
- Slot 1 H WIC/WIC/V WIC
- FastEthernet 0/0
- FastEthernet 0/1
Descriptions are provided for various components and combinations of components.
Aux port: The AUX port is a management port used for remote configuration of the device. It is most often used for the connection of a modem to manage the device when the network path has failed. Not all routers have an AUX port.
Console port: The console port allows for local configuration of the device. It is a management port and is not designed as a networking port.
User Interface DUAL U ART: Universal Asynchronous Receiver/Transmitter (U ART); two access methods through the Auxiliary and Console ports.
Compact Flash and Flash: Stores the Cisco I O S software image on either the Flash single in-line memory module (SIMM), or the PCMCIA card.
Boot ROM: Holds bootstrap, ROM monitor, and optionally scaled-down version of I O S software; shares flash memory with NV RAM.
NV RAM: Holds startup configuration; shares flash memory with Boot ROM.
CPU: Central processing unit is the brain of the computer. The CPU is where most calculations take place by interpreting the computer program instructions and processing data.
System Control ASIC: Controls the flow of data between the memory, interface, and the CPU.
S D RAM DIMM's: Hold running configuration, routing table, and other dynamic structures.
Interfaces: Two modular slots and two built-in Fast Ethernet ports.
1.1.3 Internetwork Operating System
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Internetwork Operating System
The operating system software used in Cisco routers is known as Cisco Internetwork Operating System (IOS). Like any operating system on any computer, Cisco IOS manages the hardware and software resources of the router, including memory allocation, processes, security, and file systems. Cisco IOS is a multitasking operating system that is integrated with routing, switching, internetworking, and telecommunications functions.
Although the Cisco IOS may appear to be the same on many routers, there are many different IOS images. An IOS image is a file that contains the entire IOS for that router. Cisco creates many different types of IOS images, depending upon the model of the router and the features within the IOS. Typically the more features in the IOS, the larger the IOS image, and therefore, the more flash and RAM that is required to store and load the IOS. For example, some features include the ability to run IPv6 or the ability for the router to perform NAT (Network Address Translation).
As with other operating systems Cisco IOS has its own user interface. Although some routers provide a graphical user interface (GUI), the command line interface (CLI) is a much more common method of configuring Cisco routers. The CLI is used throughout this curriculum.
Upon bootup, the startup-config file in NVRAM is copied into RAM and stored as the running-config file. IOS executes the configuration commands in the running-config. Any changes entered by the network administrator are stored in the running-config and are immediately implemented by the IOS. In this chapter, we will review some of the basic IOS commands used to configure a Cisco router. In later chapters, we will learn the commands used to configure, verify, and troubleshoot static routing and various routing protocols such as RIP, EIGRP, and OSPF.
Note: Cisco IOS and the bootup process is discussed in more detail in a later course.
1.1.3 - Internetwork Operating System
The diagram depicts multiple PC's connected to a router. The PC's have question marks on them. The following text appears on the router: Don't worry, I have the answers.
The diagram depicts multiple PC's connected to a router. The PC's have question marks on them. The following text appears on the router: Don't worry, I have the answers.
1.1.4 Router Boot-up Process
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Bootup Process
There are four major phases to the bootup process:
1. Performing the POST
2. Loading the bootstrap program
3. Locating and loading the Cisco IOS software
4. Locating and loading the startup configuration file or entering setup mode
1. Performing the POST
The Power-On Self Test (POST) is a common process that occurs on almost every computer during bootup. The POST process is used to test the router hardware. When the router is powered on, software on the ROM chip conducts the POST. During this self-test, the router executes diagnostics from ROM on several hardware components including the CPU, RAM, and NVRAM. After the POST has been completed, the router executes the bootstrap program.
2. Loading the Bootstrap Program
After the POST, the bootstrap program is copied from ROM into RAM. Once in RAM, the CPU executes the instructions in the bootstrap program. The main task of the bootstrap program is to locate the Cisco IOS and load it into RAM.
Note: At this point, if you have a console connection to the router, you will begin to see output on the screen.
3. Locating and Loading Cisco IOS
Locating the Cisco IOS software. The IOS is typically stored in flash memory, but can also be stored in other places such as a TFTP (Trivial File Transfer Protocol) server.
If a full IOS image can not be located, a scaled-down version of the IOS is copied from ROM into RAM. This version of IOS is used to help diagnose any problems and can be used to load a complete version of the IOS into RAM.
Note: A TFTP server is usually used as a backup server for IOS but it can also be used as a central point for storing and loading the IOS. IOS management and using the TFTP server is discussed in a later course.
Loading the IOS. Some of the older Cisco routers ran the IOS directly from flash, but current models copy the IOS into RAM for execution by the CPU.
Note: Once the IOS begins to load, you may see a string of pounds signs (#), as shown in the figure, while the image decompresses.
4. Locating and Loading the Configuration File
Locating the Startup Configuration File. After the IOS is loaded, the bootstrap program searches for the startup configuration file, known as startup-config, in NVRAM. This file has the previously saved configuration commands and parameters including:
- interface addresses
- routing information
- passwords
- any other configurations saved by the network administrator
Note: If the startup configuration file does not exist in NVRAM, the router may search for a TFTP server. If the router detects that it has an active link to another configured router, it sends a broadcast searching for a configuration file across the active link. This condition will cause the router to pause, but you will eventually see a console message like the following one:
%Error opening tftp://255.255.255.255/network-confg (Timed out)
%Error opening tftp://255.255.255.255/cisconet.cfg (Timed out)
Executing the Configuration File. If a startup configuration file is found in NVRAM, the IOS loads it into RAM as the running-config and executes the commands in the file, one line at a time. The running-config file contains interface addresses, starts routing processes, configures router passwords and defines other characteristics of the router.
Enter Setup Mode (Optional). If the startup configuration file can not be located, the router prompts the user to enter setup mode. Setup mode is a series of questions prompting the user for basic configuration information. Setup mode is not intended to be used to enter complex router configurations, and it is not commonly used by network administrators.
When booting a router that does not contain a startup configuration file, you will see the following question after the IOS has been loaded:
Would you like to enter the initial configuration dialog? [yes/no]: no
Setup mode will not be used in this course to configure the router. When prompted to enter setup mode, always answer no. If you answer yes and enter setup mode, you can press Ctrl-C at any time to terminate the setup process.
When setup mode is not used, the IOS creates a default running-config. The default running-config is a basic configuration file that includes the router interfaces, management interfaces, and certain default information. The default running-config does not contain any interface addresses, routing information, passwords, or other specific configuration information.
Command Line Interface
Depending on the platform and IOS, the router may ask the following question before displaying the prompt:
Would you like to terminate autoinstall? [yes]:
Press the Enter key to accept the default answer.
Router>
Note: If a startup configuration file was found, the running-config may contain a hostname and the prompt will display the hostname of the router.
Once the prompt displays, the router is now running the IOS with the current running configuration file. The network administrator can now begin using IOS commands on this router.
Note: The bootup process is discussed in more detail in a later course.
1.1.4 - Router Boot-up Process
The diagram depicts how a router boots up.
Step 1. Perform POST (from ROM).
Step 2. Execute the bootstrap loader (from ROM).
Step 3. Locate the I O S (from Flash or TFTP Server).
Step 4. Load the I O S.
Step 5. Locate the configuration file (from NV RAM, TFTP server, or console).
Step 6. Execute the configuration file (or enter Setup Mode if the configuration file cannot be found).
The various I O S messages associated with each step are displayed.
The diagram depicts how a router boots up.
Step 1. Perform POST (from ROM).
Step 2. Execute the bootstrap loader (from ROM).
Step 3. Locate the I O S (from Flash or TFTP Server).
Step 4. Load the I O S.
Step 5. Locate the configuration file (from NV RAM, TFTP server, or console).
Step 6. Execute the configuration file (or enter Setup Mode if the configuration file cannot be found).
The various I O S messages associated with each step are displayed.
Page 2:
Verifying Router Bootup Process
The show version command can be used to help verify and troubleshoot some of the basic hardware and software components of the router. The show version command displays information about the version of the Cisco IOS software currently running on the router, the version of the bootstrap program, and information about the hardware configuration, including the amount of system memory.
The output from the show version command includes:
IOS version
Cisco Internetwork Operating System Software
IOS (tm) C2600 Software (C2600-I-M), Version 12.2(28), RELEASE SOFTWARE (fc5)
This is the version of the Cisco IOS software in RAM and that is being used by the router.
ROM Bootstrap Program
ROM: System Bootstrap, Version 12.1(3r)T2, RELEASE SOFTWARE (fc1)
This shows the version of the system bootstrap software, stored in ROM memory, that was initially used to boot up the router.
Location of IOS
System image file is "flash:c2600-i-mz.122-28.bin"
This shows where the bootstrap program is located and loaded the Cisco IOS, and the complete filename of the IOS image.
CPU and Amount of RAM
cisco 2621 (MPC860) processor (revision 0x200) with 60416K/5120K bytes of memory
The first part of this line displays the type of CPU on this router. The last part of this line displays the amount of DRAM. Some series of routers, like the 2600, use a fraction of DRAM as packet memory. Packet memory is used for buffering packets.
To determine the total amount of DRAM on the router, add both numbers. In this example, the Cisco 2621 router has 60,416 KB (kilobytes) of free DRAM used for temporarily storing the Cisco IOS and other system processes. The other 5,120 KB is dedicated for packet memory. The sum of these numbers is 65,536K, or 64 megabytes (MB) of total DRAM.
Note: It may be necessary to upgrade the amount of RAM when upgrading the IOS.
Interfaces
2 FastEthernet/IEEE 802.3 interface(s)
2 Low-speed serial(sync/async) network interface(s)
This section of the output displays the physical interfaces on the router. In this example, the Cisco 2621 router has two FastEthernet interfaces and two low-speed serial interfaces.
Amount of NVRAM
32K bytes of non-volatile configuration memory.
This is the amount of NVRAM on the router. NVRAM is used to store the startup-config file.
Amount of Flash
16384K bytes of processor board System flash (Read/Write)
This is the amount of flash memory on the router. Flash is used to permanently store the Cisco IOS.
Note: It may be necessary to upgrade the amount of flash when upgrading the IOS.
Configuration Register
Configuration register is 0x2102
The last line of the show version command displays the current configured value of the software configuration register in hexadecimal. If there is a second value displayed in parentheses, it denotes the configuration register value that will be used during the next reload.
The configuration register has several uses, including password recovery. The factory default setting for the configuration register is 0x2102. This value indicates that the router will attempt to load a Cisco IOS software image from flash memory and load the startup configuration file from NVRAM.
Note: The configuration register is discussed in more detail in a later course.
1.1.4 - Router Boot-up Process
The animation depicts output from the show version command. The following information is highlighted in the output and labeled:
I O S Version: I O S C2600 Software (C2600-I-M), Version 12.2(28), RELEASE SOFTWARE (fc5)
Bootstrap Version: ROM: System Bootstrap, Version 12.1(3r)T2, RELEASE SOFTWARE (fc1)
Model and CPU: cisco 2621 (MPC860) processor (revision 0x200)
Amount of RAM: 60416K/5120K bytes of memory
Number and type of interfaces: 2 FastEthernet/IEEE 802 dot 3 interface(s)
2 Low-speed serial (sync/async) network interface(s)
Amount of NV RAM: 32K bytes of non-volatile configuration memory.
Amount of Flash: 16384K bytes of processor board system flash (Read/Write)
The animation depicts output from the show version command. The following information is highlighted in the output and labeled:
I O S Version: I O S C2600 Software (C2600-I-M), Version 12.2(28), RELEASE SOFTWARE (fc5)
Bootstrap Version: ROM: System Bootstrap, Version 12.1(3r)T2, RELEASE SOFTWARE (fc1)
Model and CPU: cisco 2621 (MPC860) processor (revision 0x200)
Amount of RAM: 60416K/5120K bytes of memory
Number and type of interfaces: 2 FastEthernet/IEEE 802 dot 3 interface(s)
2 Low-speed serial (sync/async) network interface(s)
Amount of NV RAM: 32K bytes of non-volatile configuration memory.
Amount of Flash: 16384K bytes of processor board system flash (Read/Write)
Page 3:
Use this Packet Tracer Activity to experience setup mode and investigate the show running-configuration command.
Click the Packet Tracer icon for more details.
1.1.4 - Router Boot-up Process
Link to Packet Tracer Exploration: Using Setup Mode
Use this Packet Tracer Activity to experience setup mode and investigate the show running-configuration command.
Link to Packet Tracer Exploration: Using Setup Mode
Use this Packet Tracer Activity to experience setup mode and investigate the show running-configuration command.
1.1.5 Router Interfaces
Page 1:
Management Ports
Routers have physical connectors that are used to manage the router. These connectors are known as management ports. Unlike Ethernet and serial interfaces, management ports are not used for packet forwarding. The most common management port is the console port. The console port is used to connect a terminal, or most often a PC running terminal emulator software, to configure the router without the need for network access to that router. The console port must be used during initial configuration of the router.
Another management port is the auxiliary port. Not all routers have auxiliary ports. At times the auxiliary port can be used in ways similar to a console port. It can also be used to attach a modem. Auxiliary ports will not be used in this curriculum.
The figure shows the console and AUX ports on the router.
Router Interfaces
The term interface on Cisco routers refers to a physical connector on the router whose main purpose is to receive and forward packets. Routers have multiple interfaces that are used to connect to multiple networks. Typically, the interfaces connect to various types of networks, which means that different types of media and connectors are required. Often a router will need to have different types of interfaces. For example, a router usually has FastEthernet interfaces for connections to different LANs and various types of WAN interfaces to connect a variety of serial links including T1, DSL and ISDN. The figure shows the FastEthernet and serial interfaces on the router.
Like interfaces on a PC, the ports and interfaces on a router are located on the outside of the router. Their external location allows for convenient attachment to the appropriate network cables and connectors.
Note: A single interface on a router can be used to connect to multiple networks; however, this is beyond the scope of this course and is discussed in a later course.
Like most networking devices, Cisco routers use LED indicators to provide status information. An interface LED indicates the activity of the corresponding interface. If an LED is off when the interface is active and the interface is correctly connected, this may be an indication of a problem with that interface. If an interface is extremely busy, its LED will always be on. Depending on the type of router, there may be other LEDs as well. For more information on LED displays on the 1841, see the link below.
Links
"Troubleshooting Cisco 1800 Series Routers (Modular)," http://www.cisco.com/en/US/docs/routers/access/1800/1841/hardware/installation/guide/18troub.html
1.1.5 - Router Interfaces
The diagram depicts a photograph of the back of the 1841 ISR showing the physical router interfaces. Each individual interface connects to a different network. Each interface has an IP address/mask from that network.
LAN interfaces highlighted: 10/100 Ethernet 0/0 and 10/100 Ethernet 0/1.
WAN interfaces highlighted: Serial 0 and Serial 1
The diagram depicts a photograph of the back of the 1841 ISR showing the physical router interfaces. Each individual interface connects to a different network. Each interface has an IP address/mask from that network.
LAN interfaces highlighted: 10/100 Ethernet 0/0 and 10/100 Ethernet 0/1.
WAN interfaces highlighted: Serial 0 and Serial 1
Page 2:
Interfaces Belong to Different Networks
As shown in the figure, every interface on the router is a member or host on a different IP network. Each interface must be configured with an IP address and subnet mask of a different network. Cisco IOS will not allow two active interfaces on the same router to belong to the same network.
Router interfaces can be divided into two major groups:
- LAN interfaces - such as Ethernet and FastEthernet
- WAN interfaces - such as serial, ISDN, and Frame Relay
As the name indicates, LAN interfaces are used to connect the router to the LAN, similar to how a PC Ethernet NIC is used to connect the PC to the Ethernet LAN. Like a PC Ethernet NIC, a router Ethernet interface also has a Layer 2 MAC address and participates in the Ethernet LAN in the same way as any other hosts on that LAN. For example, a router Ethernet interface participates in the ARP process for that LAN. The router maintains an ARP cache for that interface, sends ARP requests when needed, and responds with ARP replies when required.
A router Ethernet interface usually uses an RJ-45 jack that supports unshielded twisted-pair (UTP) cabling. When a router is connected to a switch, a straight-through cable is used. When two routers are connected directly through the Ethernet interfaces, or when a PC NIC is connected directly to a router Ethernet interface, a crossover cable is used.
Use the Packet Tracer Activity later in this section to test your cabling skills.
WAN Interfaces
WAN interfaces are used to connect routers to external networks, usually over a larger geographical distance. The Layer 2 encapsulation can be of different types, such as PPP, Frame Relay, and HDLC (High-Level Data Link Control). Similar to LAN interfaces, each WAN interface has its own IP address and subnet mask, which identifies it as a member of a specific network.
Note: MAC addresses are used on LAN interfaces, such as Ethernet, and are not used on WAN interfaces. However, WAN interfaces use their own Layer 2 addresses depending on the technology. Layer 2 WAN encapsulation types and addresses are covered in a later course.
Router Interfaces
The router in the figure has four interfaces. Each interface has a Layer 3 IP address and subnet mask that configures it for a different network. The Ethernet interfaces also have Layer 2 Ethernet MAC addresses.
The WAN interfaces are using different Layer 2 encapsulations. Serial 0/0/0 is using HDLC and Serial 0/0/1 is using PPP. Both of these serial point-to-point protocols use a broadcast address for the Layer 2 destination address when encapsulating the IP packet into a data link frame.
In the lab environment, you are restricted as to how many LAN and WAN interfaces you can use to configure hands-on labs. With Packet Tracer, however, you have the flexibility to create more complex network designs.
1.1.5 - Router Interfaces
The diagram depicts router interface logical representation. Interfaces are identified and the following information is provided.
HDLC Link: Interface Serial0/0/0, IP 192.168.2.1/24
P P P Link: Interface Serial0/0/1, IP 192.168.3.1/24
LAN Link 1: FastEthernet0/0, MAC: 00d0.bcb0.59a5, IP: 192.168.0.1/24
LAN Link 2: FastEthernet0/1, MAC: 0000.0c9b.d2d8, IP: 192.168.1.1/24
The diagram depicts router interface logical representation. Interfaces are identified and the following information is provided.
HDLC Link: Interface Serial0/0/0, IP 192.168.2.1/24
P P P Link: Interface Serial0/0/1, IP 192.168.3.1/24
LAN Link 1: FastEthernet0/0, MAC: 00d0.bcb0.59a5, IP: 192.168.0.1/24
LAN Link 2: FastEthernet0/1, MAC: 0000.0c9b.d2d8, IP: 192.168.1.1/24
Page 3:
Use the Packet Tracer Activity to practice selecting the correct cable to connect devices.
Click the Packet Tracer icon for more details.
1.1.5 - Router Interfaces
Link to Packet Tracer Exploration: Cabling Devices
Use the Packet Tracer Activity to practice selecting the correct cable to connect devices.
Link to Packet Tracer Exploration: Cabling Devices
Use the Packet Tracer Activity to practice selecting the correct cable to connect devices.
Page 4:
Use the Packet Tracer Activity to explore using the Physical, Config, and CLI tabs for a router.
Click the Packet Tracer icon for more details.
1.1.5 - Router Interfaces
Link to Packet Tracer Exploration: Using Packet Tracer Device Tabs
Use the Packet Tracer Activity to explore using the Physical, Config, and C L I tabs for a router.
Link to Packet Tracer Exploration: Using Packet Tracer Device Tabs
Use the Packet Tracer Activity to explore using the Physical, Config, and C L I tabs for a router.
1.1.6 Routers and the Network Layer
Page 1:
Routers and the Network Layer
The main purpose of a router is to connect multiple networks and forward packets destined either for its own networks or other networks. A router is considered a Layer 3 device because its primary forwarding decision is based on the information in the Layer 3 IP packet, specifically the destination IP address. This process is known as routing.
When a router receives a packet, it examines its destination IP address. If the destination IP address does not belong to any of the router's directly connected networks, the router must forward this packet to another router. In the figure, R1 examines the destination IP address of the packet. After searching the routing table, R1 forwards the packet onto R2. When R2 receives the packet, it also examines the packet's destination IP address. After searching its routing table, R2 forwards the packet out its directly connected Ethernet network to PC2.
When each router receives a packet, it searches its routing table to find the best match between the destination IP address of the packet and one of the network addresses in the routing table. Once a match is found, the packet is encapsulated in the layer 2 data link frame for that outgoing interface. The type of data link encapsulation depends on the type of interface, such as Ethernet or HDLC.
Eventually the packet reaches a router that is part of a network that matches the destination IP address of the packet. In this example, router R2 receives the packet from R1. R2 forwards the packet out its Ethernet interface, which belongs to the same network as the destination device, PC2.
This sequence of events is explained in more detail later in this chapter.
1.1.6 - Routers and the Network Layer
The animation depicts packet forwarding. The animation is based on the following network topology description.
Network Topology:
PC1 is connected to switch S1, which is connected to router R1. Router R1 is connected to router R2 via a WAN link. PC2 is connected to R2.
As the animation progresses, PC1 sends a packet to PC2 with IP address 192.168.3.10. Each router examines the destination IP address to correctly forward the packet.
The animation depicts packet forwarding. The animation is based on the following network topology description.
Network Topology:
PC1 is connected to switch S1, which is connected to router R1. Router R1 is connected to router R2 via a WAN link. PC2 is connected to R2.
As the animation progresses, PC1 sends a packet to PC2 with IP address 192.168.3.10. Each router examines the destination IP address to correctly forward the packet.
Page 2:
Routers Operate at Layers 1, 2, and 3
A router makes its primary forwarding decision at Layer 3, but as we saw earlier, it participates in Layer 1 and Layer 2 processes as well. After a router has examined the destination IP address of a packet and consulted its routing table to make its forwarding decision, it can forward that packet out the appropriate interface toward its destination. The router encapsulates the Layer 3 IP packet into the data portion of a Layer 2 data link frame appropriate for the exit interface. The type of frame can be an Ethernet, HDLC, or some other Layer 2 encapsulation - whatever encapsulation is used on that particular interface. The Layer 2 frame is encoded into the Layer 1 physical signals that are used to represent bits over the physical link.
To understand this process better, refer to the figure. Notice that PC1 operates at all seven layers, encapsulating the data and sending the frame out as a stream of encoded bits to R1, its default gateway.
R1 receives the stream of encoded bits on its interface. The bits are decoded and passed up to Layer 2, where R1 decapsulates the frame. The router examines the destination address of the data link frame to determine if it matches the receiving interface, including a broadcast or multicast address. If there is a match with the data portion of the frame, the IP packet is passed up to Layer 3, where R1 makes its routing decision. R1 then re-encapsulates the packet into a new Layer 2 data link frame and forwards it out the outbound interface as a stream of encoded bits.
R2 receives the stream of bits, and the process repeats itself. R2 decapsulates the frame and passes the data portion of the frame, the IP packet, to Layer 3 where R2 makes its routing decision. R2 then re-encapsulates the packet into a new Layer 2 data link frame and forwards it out the outbound interface as a stream of encoded bits.
This process is repeated once again by router R3, which forwards the IP packet, encapsulated inside a data link frame and encoded as bits, to PC2.
Each router in the path from source to destination performs this same process of decapsulation, searching the routing table, and then re-encapsulation. This process is important to your understanding of how routers participate in networks. Therefore, we will revisit this discussion in more depth in a later section.
1.1.6 - Routers and the Network Layer
The diagram depicts how routers operate at Layers 1, 2, and 3 of the O S I Model. The diagram is based on the following network topology description.
Network Topology:
PC1, with IP address 192.168.1.10, is connected to router R1. Router R1 is connected to router R2. R2 is connected to router R3. PC2 with IP address 192.168.4.10 is connected to R3.
In the diagram, the seven-layer O S I stack is shown for each PC, indicating that computers such as PC's operate at all seven O S I layers. Only the first three layers of the stack (Physical, Data Link, and Network) are shown for each router, indicating that routers operate at the first three O S I layers.
Red arrows indicate the flow through the O S I layers in the diagram. PC1 sends a packet to PC2. The packet travels down though all seven O S I layers from the Application Layer to the Physical Layer.
At each router, the packet only flows up from the Physical Layer to the Network Layer. When the packet reaches PC2, it travels up through all seven O S I layers from the Physical Layer to the Application Layer.
The diagram depicts how routers operate at Layers 1, 2, and 3 of the O S I Model. The diagram is based on the following network topology description.
Network Topology:
PC1, with IP address 192.168.1.10, is connected to router R1. Router R1 is connected to router R2. R2 is connected to router R3. PC2 with IP address 192.168.4.10 is connected to R3.
In the diagram, the seven-layer O S I stack is shown for each PC, indicating that computers such as PC's operate at all seven O S I layers. Only the first three layers of the stack (Physical, Data Link, and Network) are shown for each router, indicating that routers operate at the first three O S I layers.
Red arrows indicate the flow through the O S I layers in the diagram. PC1 sends a packet to PC2. The packet travels down though all seven O S I layers from the Application Layer to the Physical Layer.
At each router, the packet only flows up from the Physical Layer to the Network Layer. When the packet reaches PC2, it travels up through all seven O S I layers from the Physical Layer to the Application Layer.
1.2 CLI Configuration and Addressing
1.2.1 Implementing Basic Addressing SchemesPage 1:
When designing a new network or mapping an existing network, document the network. At a minimum, the documentation should include a topology diagram that indicates the physical connectivity and an addressing table that lists all of the following information:
- Device names
- Interfaces used in the design
- IP addresses and subnet masks
- Default gateway addresses for end devices, such as PCs
The figure shows a network topology with the devices interconnected and configured with IP addresses. Under the topology is a table used to document the network. The table is partially populated with the data documenting the network (devices, IP addresses, subnet masks, and interfaces).
Router R1 and host PC1 are already documented. Finish populating the table and the blank spaces on the diagram dragging the pool of IP addresses shown below the table to the correct locations.
1.2.1 - Implementing Basic Addressing Schemes
The diagram depicts an activity in which you must assign an address to the proper device and interface. The activity is titled: Documenting an Addressing Scheme.
Note: You might want to contact your instructor for assistance with this activity.
This activity pertains to the following network topology.
Network Topology:
LAN A PC1 is connected to switch S1, which is connected to router R1 via interface FA0/0. Router R1 interface S0/0/0 is connected to router R2 interface S0/0/0 via a WAN link. LAN B PC2 is connected to switch S2, which is connected to R2 interface FA0/0.
LAN A network address: 192.168.1.0/24
LAN B network address: 192.168.3.0/24
WAN network address: 192.168.2.0/24
Fill in the blanks with the correct address from the listing below:
Router R1 interfaces and addresses:
Interface: FA0/0
IP address: 192.168.1.1
Subnet mask: 255.255.255.0
Interface: S0/0/0
IP address: 192.168.2.1
Subnet mask: 255.255.255.0
Router R2 interfaces and addresses:
Interface: FA0/0
IP address: BLANK
Subnet mask: 255.255.255.0
Interface: S0/0/0
IP address: BLANK
Subnet mask: 255.255.255.0
PC1 addresses:
IP address: 192.168.1.10
Subnet mask 255.255.255.0
Default gateway: 192.168.1.1
PC2 addresses:
IP address: BLANK
Subnet mask 255.255.255.0
Default gateway: BLANK
IP addresses to choose from:
192.168.2.2
192.168.1.1
192.168.2.1
192.168.3.1
192.168.3.10
192.168.2.2
192.168.1.10
192.168.3.1
192.168.3.1
192.168.3.10
The diagram depicts an activity in which you must assign an address to the proper device and interface. The activity is titled: Documenting an Addressing Scheme.
Note: You might want to contact your instructor for assistance with this activity.
This activity pertains to the following network topology.
Network Topology:
LAN A PC1 is connected to switch S1, which is connected to router R1 via interface FA0/0. Router R1 interface S0/0/0 is connected to router R2 interface S0/0/0 via a WAN link. LAN B PC2 is connected to switch S2, which is connected to R2 interface FA0/0.
LAN A network address: 192.168.1.0/24
LAN B network address: 192.168.3.0/24
WAN network address: 192.168.2.0/24
Fill in the blanks with the correct address from the listing below:
Router R1 interfaces and addresses:
Interface: FA0/0
IP address: 192.168.1.1
Subnet mask: 255.255.255.0
Interface: S0/0/0
IP address: 192.168.2.1
Subnet mask: 255.255.255.0
Router R2 interfaces and addresses:
Interface: FA0/0
IP address: BLANK
Subnet mask: 255.255.255.0
Interface: S0/0/0
IP address: BLANK
Subnet mask: 255.255.255.0
PC1 addresses:
IP address: 192.168.1.10
Subnet mask 255.255.255.0
Default gateway: 192.168.1.1
PC2 addresses:
IP address: BLANK
Subnet mask 255.255.255.0
Default gateway: BLANK
IP addresses to choose from:
192.168.2.2
192.168.1.1
192.168.2.1
192.168.3.1
192.168.3.10
192.168.2.2
192.168.1.10
192.168.3.1
192.168.3.1
192.168.3.10
Page 2:
Use the Packet Tracer Activity to connect the devices. Configure the device names to match the figure and use the Place Note feature to add network address labels.
Click the Packet Tracer icon for more details.
1.2.1 - Implementing Basic Addressing Schemes
Link to Packet Tracer Exploration: Connecting and Identifying Devices
Use the Packet Tracer activity to connect the devices. Configure the device names to match the figure and use the Place Note feature to add network address labels.
Link to Packet Tracer Exploration: Connecting and Identifying Devices
Use the Packet Tracer activity to connect the devices. Configure the device names to match the figure and use the Place Note feature to add network address labels.
1.2.2 Basic Router Configuration
Page 1:
Basic Router Configuration
When configuring a router, certain basic tasks are performed including:
- Naming the router
- Setting passwords
- Configuring interfaces
- Configuring a banner
- Saving changes on a router
- Verifying basic configuration and router operations
The first prompt appears at user mode. User mode allows you to view the state of the router, but does not allow you to modify its configuration. Do not confuse the term "user" as used in user mode with users of the network. User mode is intended for the network technicians, operators, and engineers who have the responsibility to configure network devices.
Router>
The enable command is used to enter the privileged EXEC mode. This mode allows the user to make configuration changes on the router. The router prompt will change from a ">" to a "#" in this mode.
Router>enable
Router#
Hostnames and Passwords
The figure shows the basic router configuration command syntax used to configure R1 in the following example. You can open Packet Tracer Activity 1.2.2 and follow along or wait until the end of this section to open it.
First, enter the global configuration mode.
Router#config t
Next, apply a unique hostname to the router.
Router(config)#hostname R1
R1(config)#
Now, configure a password that is to be used to enter privileged EXEC mode. In our lab environment, we will use the password class. However, in production environments, routers should have strong passwords. See the links at the end of this section for more information on creating and using strong passwords.
Router(config)#enable secret class
Next, configure the console and Telnet lines with the password cisco. Once again, the password cisco is used only in our lab environment. The command login enables password checking on the line. If you do not enter the command login on the console line, the user will be granted access to the line without entering a password.
R1(config)#line console 0
R1(config-line)#password cisco
R1(config-line)#login
R1(config-line)#exit
R1(config)#line vty 0 4
R1(config-line)#password cisco
R1(config-line)#login
R1(config-line)#exit
Configuring a Banner
From the global configuration mode, configure the message-of-the-day (motd) banner. A delimiting character, such as a "#" is used at the beginning and at the end of the message. The delimiter allows you to configure a multiline banner, as shown here.
R1(config)#banner motd #
Enter TEXT message. End with the character '#'.
******************************************
WARNING!! Unauthorized Access Prohibited!!
******************************************
#
Configuring an appropriate banner is part of a good security plan. At a very minimum, a banner should warn against unauthorized access. Never configure a banner that "welcomes" an unauthorized user.
Links
For discussions about using strong passwords, see:
"Cisco Response to Dictionary Attacks on Cisco LEAP," at http://www.cisco.com/en/US/products/hw/wireless/ps430/prod_bulletin09186a00801cc901.html#wp1002291
"Strong passwords: How to create and use them," at http://www.microsoft.com/athome/security/privacy/password.mspx
1.2.2 - Basic Router Configuration
The diagram depicts basic router configuration command syntax.
Naming the router:
Router(config)#hostname name
Setting Passwords
Router(config)#enable secret password
Router(config)#line console 0
Router(config)#password password
Router(config)#login
Router(config)#line v t y 0 4
Router(config)#password password
Router(config)#enable secret password
Router(config)#login
Configuring a message-of-the-day banner:
Router(config)#banner m o t d #message#
The diagram depicts basic router configuration command syntax.
Naming the router:
Router(config)#hostname name
Setting Passwords
Router(config)#enable secret password
Router(config)#line console 0
Router(config)#password password
Router(config)#login
Router(config)#line v t y 0 4
Router(config)#password password
Router(config)#enable secret password
Router(config)#login
Configuring a message-of-the-day banner:
Router(config)#banner m o t d #message#
Page 2:
Router Interface Configuration
You will now configure the individual router interfaces with IP addresses and other information. First, enter the interface configuration mode by specifying the interface type and number. Next, configure the IP address and subnet mask:
R1(config)#interface Serial0/0/0
R1(config-if)#ip address 192.168.2.1 255.255.255.0
It is good practice to configure a description on each interface to help document the network information. The description text is limited to 240 characters. On production networks a description can be helpful in troubleshooting by providing information about the type of network that the interface is connected to and if there are any other routers on that network. If the interface connects to an ISP or service carrier, it is helpful to enter the third party connection and contact information; for example:
Router(config-if)#description Ciruit#VBN32696-123 (help desk:1-800-555-1234)
In lab environments, enter a simple description that will help in troubleshooting situations; for example:
R1(config-if)#description Link to R2
After configuring the IP address and description, the interface must be activated with the no shutdown command. This is similar to powering on the interface. The interface must also be connected to another device (a hub, a switch, another router, etc.) for the Physical layer to be active.
Router(config-if)#no shutdown
Note: When cabling a point-to-point serial link in our lab environment, one end of the cable is marked DTE and the other end is marked DCE. The router that has the DCE end of the cable connected to its serial interface will need the additional clock rate command configured on that serial interface. This step is only necessary in a lab environment and will be explained in more detail in Chapter 2, "Static Routing".
R1(config-if)#clock rate 64000
Repeat the interface configuration commands on all other interfaces that need to be configured. In our topology example, the FastEthernet interface needs to be configured.
R1(config)#interface FastEthernet0/0
R1(config-if)#ip address 192.168.1.1 255.255.255.0
R1(config-if)#description R1 LAN
R1(config-if)#no shutdown
Each Interface Belongs to a Different Network
At this point, note that each interface must belong to a different network. Although the IOS allows you to configure an IP address from the same network on two different interfaces, the router will not activate the second interface.
For example, what if you attempt to configure the FastEthernet 0/1 interface on R1 with an IP address on the 192.168.1.0/24 network? FastEthernet 0/0 has already been assigned an address on that same network. If you attempt to configure another interface, FastEthernet 0/1, with an IP address that belongs to the same network, you will get the following message:
R1(config)#interface FastEthernet0/1
R1(config-if)#ip address 192.168.1.2 255.255.255.0
192.168.1.0 overlaps with FastEthernet0/0
If there is an attempt to enable the interface with the no shutdown command, the following message will appear:
R1(config-if)#no shutdown
192.168.1.0 overlaps with FastEthernet0/0
FastEthernet0/1: incorrect IP address assignment
Notice that the output from the show ip interface brief command shows that the second interface configured for the 192.168.1.0/24 network, FastEthernet 0/1, is still down.
R1#show ip interface brief
1.2.2 - Basic Router Configuration
The diagram depicts additional basic router configuration command syntax.
Configuring an interface:
Router(config)#interface type number
Router(config)#i p address address mask
Router(config)#description description
Saving changes on a router:
Router(config)#copy running-config startup-config
Examining the output of show commands:
Router(config)#show running-config
Router(config)#show i p route
Router(config)#show interface brief
Router(config)#show interfaces
The diagram depicts additional basic router configuration command syntax.
Configuring an interface:
Router(config)#interface type number
Router(config)#i p address address mask
Router(config)#description description
Saving changes on a router:
Router(config)#copy running-config startup-config
Examining the output of show commands:
Router(config)#show running-config
Router(config)#show i p route
Router(config)#show interface brief
Router(config)#show interfaces
Page 3:
Verifying Basic Router Configuration
Currently in the example, all of the previous basic router configuration commands have been entered and were immediately stored in the running configuration file of R1. The running-config file is stored in RAM and is the configuration file used by IOS. The next step is to verify the commands entered by displaying the running configuration with the following command:
R1#show running-config
Now that the basic configuration commands have been entered, it is important to save the running-config to the nonvolatile memory, the NVRAM of the router. That way, in case of a power outage or an accidental reload, the router will be able to boot with the current configuration. After the router's configuration has been completed and tested, it is important to save the running-config to the startup-config as the permanent configuration file.
R1#copy running-config startup-config
After applying and saving the basic configuration, you can use several commands to verify that you have correctly configured the router. Click the appropriate button in the figure to see a listing of each command's output. All of these commands are discussed in detail in later chapters. For now, begin to become familiar with the output.
R1#show running-config
This command displays the current running configuration that is stored in RAM. With a few exceptions, all configuration commands that were used will be entered into the running-config and implemented immediately by the IOS.
R1#show startup-config
This command displays the startup configuration file stored in NVRAM. This is the configuration that the router will use on the next reboot. This configuration does not change unless the current running configuration is saved to NVRAM with the copy running-config startup-config command. Notice in the figure that the startup configuration and the running configuration are identical. They are identical because the running configuration has not changed since the last time it was saved. Also notice that the show startup-config command also displays how many bytes of NVRAM the saved configuration is using.
R1#show ip route
This command displays the routing table that the IOS is currently using to choose the best path to its destination networks. At this point, R1 only has routes for its directly connected networks via its own interfaces.
R1#show interfaces
This command displays all of the interface configuration parameters and statistics. Some of this information is discussed later in the curriculum and in CCNP.
R1#show ip interface brief
This command displays abbreviated interface configuration information, including IP address and interface status. This command is a useful tool for troubleshooting and a quick way to determine the status of all router interfaces.
1.2.2 - Basic Router Configuration
The diagram depicts commands to verify basic router configuration. The diagram shows a network topology described as follows.
Network Topology:
LAN A, with a network address of 192.168.1.0/24, is connected to switch S1. S1 is connected to router R1. R1 is connected to router R2 via a WAN link with a network address of 192.168.2.0/24. R2 is connected to switch S1 and LAN B with a network address of 192.168.3.0/24.
R1#show running-config
This command verifies the commands entered by displaying the running configuration.
R1#show startup-config
This command displays the startup configuration file stored in NV RAM. This is the configuration the router uses on the next reboot.
R1#show interfaces
This command displays all the configuration parameters and statistics.
R1#show i p route
This command displays the routing table that the I O S is currently using to choose the best path to its destination networks. The output displayed shows R1 only having routes to its directly connected networks via its own interfaces.
R1#show interfaces brief
This command displays abbreviated interface configuration information, including IP address and interface status.
The diagram depicts commands to verify basic router configuration. The diagram shows a network topology described as follows.
Network Topology:
LAN A, with a network address of 192.168.1.0/24, is connected to switch S1. S1 is connected to router R1. R1 is connected to router R2 via a WAN link with a network address of 192.168.2.0/24. R2 is connected to switch S1 and LAN B with a network address of 192.168.3.0/24.
R1#show running-config
This command verifies the commands entered by displaying the running configuration.
R1#show startup-config
This command displays the startup configuration file stored in NV RAM. This is the configuration the router uses on the next reboot.
R1#show interfaces
This command displays all the configuration parameters and statistics.
R1#show i p route
This command displays the routing table that the I O S is currently using to choose the best path to its destination networks. The output displayed shows R1 only having routes to its directly connected networks via its own interfaces.
R1#show interfaces brief
This command displays abbreviated interface configuration information, including IP address and interface status.
Page 4:
Use the Packet Tracer Activity to practice basic router configuration and verification commands.
Click the Packet Tracer icon for more details.
1.2.2 - Basic Router Configuration
Link to Packet Tracer Exploration: Configure and Verify R1
Use the Packet Tracer Activity to practice basic router configuration and verification commands.
Link to Packet Tracer Exploration: Configure and Verify R1
Use the Packet Tracer Activity to practice basic router configuration and verification commands.
1.3 Building the Routing Table
1.3.1 Introducing the Routing TablePage 1:
Introducing the Routing Table
The primary function of a router is to forward a packet toward its destination network, which is the destination IP address of the packet. To do this, a router needs to search the routing information stored in its routing table.
A routing table is a data file in RAM that is used to store route information about directly connected and remote networks. The routing table contains network/next hop associations. These associations tell a router that a particular destination can be optimally reached by sending the packet to a specific router that represents the "next hop" on the way to the final destination. The next hop association can also be the outgoing or exit interface to the final destination.
The network/exit-interface association can also represent the destination network address of the IP packet. This association occurs on the router's directly connected networks.
A directly connected network is a network that is directly attached to one of the router interfaces. When a router interface is configured with an IP address and subnet mask, the interface becomes a host on that attached network. The network address and subnet mask of the interface, along with the interface type and number, are entered into the routing table as a directly connected network. When a router forwards a packet to a host, such as a web server, that host is on the same network as a router's directly connected network.
A remote network is a network that is not directly connected to the router. In other words, a remote network is a network that can only be reached by sending the packet to another router. Remote networks are added to the routing table using either a dynamic routing protocol or by configuring static routes. Dynamic routes are routes to remote networks that were learned automatically by the router, using a dynamic routing protocol. Static routes are routes to networks that a network administrator manually configured.
Note: The routing table-with its directly-connected networks, static routes, and dynamic routes-will be introduced in the following sections and discussed in even greater detail throughout this course.
The following analogies may help clarify the concept of connected, static, and dynamic routes:
- Directly Connected Routes - To visit a neighbor, you only have to go down the street on which you already live. This path is similar to a directly-connected route because the "destination" is available directly through your "connected interface," the street.
- Static Routes - A train uses the same railroad tracks every time for a specified route. This path is similar to a static route because the path to the destination is always the same.
- Dynamic Routes - When driving a car, you can "dynamically" choose a different path based on traffic, weather, or other conditions. This path is similar to a dynamic route because you can choose a new path at many different points on your way to the destination.
As shown in the figure the routing table is displayed with the show ip route command. At this point, there have not been any static routes configured nor any dynamic routing protocol enabled. Therefore, the routing table for R1 only shows the router's directly connected networks. For each network listed in the routing table, the following information is included:
- C - The information in this column denotes the source of the route information, directly connected network, static route or a dynamic routing protocol. The C represents a directly connected route.
- 192.168.1.0/24 - This is the network address and subnet mask of the directly connected or remote network. In this example, both entries in the routing table, 192.168.1./24 and 192.168.2.0/24, are directly connected networks.
- FastEthernet 0/0 - The information at the end of the route entry represents the exit interface and/or the IP address of the next-hop router. In this example, both FastEthernet 0/0 and Serial0/0/0 are the exit interfaces used to reach these networks.
PCs also have a routing table. In the figure, you can see the route print command output. The command reveals the configured or acquired default gateway, connected, loopback, multicast, and broadcast networks. The output from route print command will not be analyzed during this course. It is shown here to emphasize the point that all IP configured devices should have a routing table.
1.3.1 - Introducing the Routing Table
The diagram depicts routing tables and the information that they contain using the show i p route command. The diagram shows a network topology described as follows.
Network Topology:
LAN A, with a network address of 192.168.1.0/24, is connected to switch S1. S1 is connected to router R1. R1 is connected to router R2 via a WAN link with a network address of 192.168.2.0/24. R2 is connected to switch S1 and LAN B with a network address of 192.168.3.0/24.
The R1 interface FA0/0 IP address is 192.168.1.1.
The R1 interface S0/0/0 IP address is 192.168.2.1.
The R2 interface FA0/0 IP address is 192.168.3.1.
The R2 interface S0/0/0 IP address is 192.168.2.2.
Information highlighted in the show i p route command output includes the following. In the output, C equals connected.
C 192.168.1.0/24 is directly connected, FastEthernet0/0
C 192.168.2.0/24 is directly connected, Serial0/0/0
Output is also shown depicting the PC's routing table using the PC route print command. This command shows the default route gateway and active route network destination as follows.
Network Destination: 0.0.0.0
Netmask: 0.0.0.0
Gateway: 192.168.1.1
Interface: 192.168.1.1
Metric: 10
The diagram depicts routing tables and the information that they contain using the show i p route command. The diagram shows a network topology described as follows.
Network Topology:
LAN A, with a network address of 192.168.1.0/24, is connected to switch S1. S1 is connected to router R1. R1 is connected to router R2 via a WAN link with a network address of 192.168.2.0/24. R2 is connected to switch S1 and LAN B with a network address of 192.168.3.0/24.
The R1 interface FA0/0 IP address is 192.168.1.1.
The R1 interface S0/0/0 IP address is 192.168.2.1.
The R2 interface FA0/0 IP address is 192.168.3.1.
The R2 interface S0/0/0 IP address is 192.168.2.2.
Information highlighted in the show i p route command output includes the following. In the output, C equals connected.
C 192.168.1.0/24 is directly connected, FastEthernet0/0
C 192.168.2.0/24 is directly connected, Serial0/0/0
Output is also shown depicting the PC's routing table using the PC route print command. This command shows the default route gateway and active route network destination as follows.
Network Destination: 0.0.0.0
Netmask: 0.0.0.0
Gateway: 192.168.1.1
Interface: 192.168.1.1
Metric: 10
1.3.2 Directly-Connected Networks
Page 1:
Adding a Connected Network to the Routing Table
As stated in the previous section, when a router's interface is configured with an IP address and subnet mask, that interface becomes a host on that network. For example, when the FastEthernet 0/0 interface on R1in the figure is configured with the IP address 192.168.1.1 and the subnet mask 255.255.255.0, the FastEthernet 0/0 interface becomes a member of the 192.168.1.0/24 network. Hosts that are attached to the same LAN, like PC1, are also configured with an IP address that belongs to the 192.168.1.0/24 network.
When a PC is configured with a host IP address and subnet mask, the PC uses the subnet mask to determine what network it now belongs to. This is done by the operating system ANDing the host IP address and subnet mask. A router uses the same logic when an interface is configured.
A PC is normally configured with a single host IP address because it only has a single network interface, usually an Ethernet NIC. Routers have multiple interfaces; therefore, each interface must be a member of a different network. In the figure, R1 is a member of two different networks: 192.168.1.0/24 and 192.168.2.0/24. Router R2 is also a member of two networks: 192.168.2.0/24 and 192.168.3.0/24.
After the router's interface is configured and the interface is activated with the no shutdown command, the interface must receive a carrier signal from another device (router, switch, hub, etc.) before the interface state is considered "up." Once the interface is "up," the network of that interface is added to the routing table as a directly connected network.
Before any static or dynamic routing is configured on a router, the router only knows about its own directly connected networks. These are the only networks that are displayed in the routing table until static or dynamic routing is configured. Directly connected networks are of prime importance for routing decisions. Static and dynamic routes cannot exist in the routing table without a router's own directly connected networks. The router cannot send packets out an interface if that interface is not enabled with an IP address and subnet mask, just as a PC cannot send IP packets out its Ethernet interface if that interface is not configured with an IP address and subnet mask.
Note: The process of configuring router interfaces and adding network address to the routing table are discussed in the following chapter.
1.3.2 - Directly Connected Networks
The diagram depicts connected route entries in the routing table. The network topology is identical to the network topology described in 1.3.1 diagram 1.
The highlighted output for the show i p route command includes:
C 192.168.1.0/24 is directly connected, FastEthernet0/0
C 192.168.2.0/24 is directly connected, Serial0/0/0
The diagram depicts connected route entries in the routing table. The network topology is identical to the network topology described in 1.3.1 diagram 1.
The highlighted output for the show i p route command includes:
C 192.168.1.0/24 is directly connected, FastEthernet0/0
C 192.168.2.0/24 is directly connected, Serial0/0/0
Page 2:
Use the Packet Tracer Activity to learn how the IOS installs and removes directly connected routes.
Click the Packet Tracer icon for more details.
1.3.2 - Directly Connected Networks
Link to Packet Tracer Exploration: Directly Connected Routes
Use the Packet Tracer Activity to learn how the I O S installs and removes directly connected routes.
Link to Packet Tracer Exploration: Directly Connected Routes
Use the Packet Tracer Activity to learn how the I O S installs and removes directly connected routes.
1.3.3 Static Routing
Page 1:
Static Routing
Remote networks are added to the routing table either by configuring static routes or enabling a dynamic routing protocol. When the IOS learns about a remote network and the interface that it will use to reach that network, it adds that route to the routing table as long as the exit interface is enabled.
A static route includes the network address and subnet mask of the remote network, along with the IP address of the next-hop router or exit interface. Static routes are denoted with the code S in the routing table as shown in the figure. Static routes are examined in detail in the next chapter.
When to Use Static Routes
Static routes should be used in the following cases:
- A network consists of only a few routers. Using a dynamic routing protocol in such a case does not present any substantial benefit. On the contrary, dynamic routing may add more administrative overhead.
- A network is connected to the Internet only through a single ISP. There is no need to use a dynamic routing protocol across this link because the ISP represents the only exit point to the Internet.
- A large network is configured in a hub-and-spoke topology. A hub-and-spoke topology consists of a central location (the hub) and multiple branch locations (spokes), with each spoke having only one connection to the hub. Using dynamic routing would be unnecessary because each branch has only one path to a given destination-through the central location.
1.3.3 - Static Routing
The diagram depicts connected and static routes. The network topology is identical to the network topology described in 1.3.1 diagram 1.
The highlighted output for the show i p route command includes the following. S in the output equals static.
S 192.168.3.0/24 [1/0] via 192.168.2.2
The diagram depicts connected and static routes. The network topology is identical to the network topology described in 1.3.1 diagram 1.
The highlighted output for the show i p route command includes the following. S in the output equals static.
S 192.168.3.0/24 [1/0] via 192.168.2.2
Page 2:
Use the Packet Tracer Activity to learn how the IOS installs and removes static routes.
Click the Packet Tracer icon for more details.
1.3.3 - Static Routing
Link to Packet Tracer Exploration: Static Routing
Use the Packet Tracer Activity to learn how the I O S installs and removes static routes.
Link to Packet Tracer Exploration: Static Routing
Use the Packet Tracer Activity to learn how the I O S installs and removes static routes.
1.3.4 Dynamic Routing
Page 1:
Dynamic Routing
Remote networks can also be added to the routing table by using a dynamic routing protocol. In the figure, R1 has automatically learned about the 192.168.4.0/24 network from R2 through the dynamic routing protocol, RIP (Routing Information Protocol). RIP was one of the first IP routing protocols and will be fully discussed in later chapters.
Note: R1's routing table in the figure shows that R1 has learned about two remote networks: one route that dynamically used RIP and a static route that was configured manually. This is an example of how routing tables can contain routes learned dynamically and configured statically and is not necessarily representative of the best configuration for this network.
Dynamic routing protocols are used by routers to share information about the reachability and status of remote networks. Dynamic routing protocols perform several activities, including:
- Network discovery
- Updating and maintaining routing tables
Network discovery is the ability of a routing protocol to share information about the networks that it knows about with other routers that are also using the same routing protocol. Instead of configuring static routes to remote networks on every router, a dynamic routing protocol allows the routers to automatically learn about these networks from other routers. These networks - and the best path to each network - are added to the router's routing table and denoted as a network learned by a specific dynamic routing protocol.
Maintaining Routing Tables
After the initial network discovery, dynamic routing protocols update and maintain the networks in their routing tables. Dynamic routing protocols not only make a best path determination to various networks, they will also determine a new best path if the initial path becomes unusable (or if the topology changes). For these reasons, dynamic routing protocols have an advantage over static routes. Routers that use dynamic routing protocols automatically share routing information with other routers and compensate for any topology changes without involving the network administrator.
IP Routing Protocols
There are several dynamic routing protocols for IP. Here are some of the more common dynamic routing protocols for routing IP packets:
- RIP (Routing Information Protocol)
- IGRP (Interior Gateway Routing Protocol)
- EIGRP (Enhanced Interior Gateway Routing Protocol)
- OSPF (Open Shortest Path First)
- IS-IS (Intermediate System-to-Intermediate System)
- BGP (Border Gateway Protocol)
Once again, remember that in most cases, routers contain a combination of static routes and dynamic routes in the routing tables. Dynamic routing protocols will be discussed in more detail in Chapter 3, "Introduction to Dynamic Routing Protocols."
1.3.4 - Dynamic Routing
The diagram depicts connected, static, and dynamic routes.
Network Topology:
The diagram shows a network topology is identical to the network topology described in 1.3.1 diagram 1, except that R2 now has the additional interface, FA0/1, connected to another LAN, with a network address of 192.168.4.0/24.
Highlighted output for the show i p route command includes the following. In the output, R equals RIP.
R 192.168.4.0/24 [120/1] via 192.168.2.2, 00:00:20, Serial0/0/0
The diagram depicts connected, static, and dynamic routes.
Network Topology:
The diagram shows a network topology is identical to the network topology described in 1.3.1 diagram 1, except that R2 now has the additional interface, FA0/1, connected to another LAN, with a network address of 192.168.4.0/24.
Highlighted output for the show i p route command includes the following. In the output, R equals RIP.
R 192.168.4.0/24 [120/1] via 192.168.2.2, 00:00:20, Serial0/0/0
Page 2:
Use the Packet Tracer Activity to learn how the IOS installs and removes dynamic routes.
Click the Packet Tracer icon for more details.
1.3.4 - Dynamic Routing
Link to Packet Tracer Exploration: Dynamic Routing
Use the Packet Tracer Activity to learn how the I O S installs and removes dynamic routes.
Link to Packet Tracer Exploration: Dynamic Routing
Use the Packet Tracer Activity to learn how the I O S installs and removes dynamic routes.
1.3.5 Routing Table Principles
Page 1:
Routing Table Principles
At times in this course we will refer to three principles regarding routing tables that will help you understand, configure, and troubleshoot routing issues. These principles are from Alex Zinin's book, Cisco IP Routing.
1. Every router makes its decision alone, based on the information it has in its own routing table.
2. The fact that one router has certain information in its routing table does not mean that other routers have the same information.
3. Routing information about a path from one network to another does not provide routing information about the reverse, or return, path.
What is the effect of these principles? Let's look at the example in the figure.
1. After making its routing decision, router R1 forwards the packet destined for PC2 to router R2. R1 only knows about the information in its own routing table, which indicates that router R2 is the next-hop router. R1 does not know whether or not R2 actually has a route to the destination network.
2. It is the responsibility of the network administrator to make sure that all routers within their control have complete and accurate routing information so that packets can be forwarded between any two networks. This can be done using static routes, a dynamic routing protocol, or a combination of both.
3. Router R2 was able to forward the packet toward PC2's destination network. However, the packet from PC2 to PC1 was dropped by R2. Although R2 has information in its routing table about the destination network of PC2, we do not know if it has the information for the return path back to PC1's network.
Asymmetric Routing
Because routers do not necessarily have the same information in their routing tables, packets can traverse the network in one direction, using one path, and return via another path. This is called asymmetric routing. Asymmetric routing is more common in the Internet, which uses the BGP routing protocol than it is in most internal networks.
This example implies that when designing and troubleshooting a network, the network administrator should check the following routing information:
- Is there a path from source to destination available in both directions?
- Is the path taken in both directions the same path? (Asymmetrical routing is not uncommon, but sometimes can pose additional issues.)
1.3.5 - Routing Table Principles
The animation depicts routing principle 3 in action using the following network topology.
Network Topology:
PC1 is connected to router R1. Router R1 is connected to router R2 via a WAN link. Router R2 is connected to router R3 via a WAN link. PC2 is connected to router R3.
As the animation progresses:
1. PC1 sends a ping to PC2.
2. R1 has a route to PC2's network.
3. R2 has a route to PC2's network.
4. R3 is directly connected to PC2's network.
5. PC2 sends a reply ping to PC1.
6. R3 has a route to PC1's network.
7. R2 does not have a route to PC1's network.
8. The packet is dropped at R2.
The animation depicts routing principle 3 in action using the following network topology.
Network Topology:
PC1 is connected to router R1. Router R1 is connected to router R2 via a WAN link. Router R2 is connected to router R3 via a WAN link. PC2 is connected to router R3.
As the animation progresses:
1. PC1 sends a ping to PC2.
2. R1 has a route to PC2's network.
3. R2 has a route to PC2's network.
4. R3 is directly connected to PC2's network.
5. PC2 sends a reply ping to PC1.
6. R3 has a route to PC1's network.
7. R2 does not have a route to PC1's network.
8. The packet is dropped at R2.
Page 2:
Use the Packet Tracer Activity to investigate a fully-converged network with connected, static, and dynamic routing.
Click the Packet Tracer icon for more details.
1.3.5 - Routing Table Principles
Link to Packet Tracer Exploration: Routing Table Principles
Use the Packet Tracer Activity to investigate a fully converged network with connected, static, and dynamic routing.
Link to Packet Tracer Exploration: Routing Table Principles
Use the Packet Tracer Activity to investigate a fully converged network with connected, static, and dynamic routing.
1.4 Path Determination and Switching Functions
1.4.1 Packet Fields and Frame FieldsPage 1:
Packet Fields and Frame Fields
As we discussed previously, routers make their primary forwarding decision by examining the destination IP address of a packet. Before sending a packet out the proper exit interface, the IP packet needs to be encapsulated into a Layer 2 data link frame. Later in this section we will follow an IP packet from source to destination, examining the encapsulation and decapsulation process at each router. But first, we will review the format of a Layer 3 IP packet and a Layer 2 Ethernet frame.
Internet Protocol (IP) Packet Format
The Internet Protocol specified in RFC 791 defines the IP packet format. The IP packet header has specific fields that contain information about the packet and about the sending and receiving hosts. Below is a list of the fields in the IP header and a brief description for each one. You should already be familiar with destination IP address, source IP address, version, and Time To Live (TTL) fields. The other fields are important but are outside the scope of this course.
- Version - Version number (4 bits); predominant version is IP version 4 (IPv4)
- IP header length - Header length in 32-bit words (4 bits)
- Precedence and type of service - How the datagram should be handled (8 bits); the first 3 bits are precedence bits (this use has been superseded by Differentiated Services Code Point [DSCP], which uses the first 6 bits [last 2 reserved])
- Packet length - Total length (header + data) (16 bits)
- Identification - Unique IP datagram value (16 bits)
- Flags - Controls fragmenting (3 bits)
- Fragment offset - Supports fragmentation of datagrams to allow differing maximum transmission units (MTUs) in the Internet (13 bits)
- Time to Live (TTL) - Identifies how many routers can be traversed by the datagram before being dropped (8 bits)
- Protocol - Upper-layer protocol sending the datagram (8 bits)
- Header checksum - Integrity check on the header (16 bits)
- Source IP address - 32-bit source IP address (32 bits)
- Destination IP address - 32-bit destination IP address (32 bits)
- IP options - Network testing, debugging, security, and others (0 or 32 bits, if any)
1.4.1 - Packet Fields and Frame Fields
The diagram depicts the fields of an IP packet in the following sequence:
1. Version
2. IHL
3. Service Type
4. Packet Length
5. Identification
6. Flag
7. Fragment Offset
8. Time to Live
9. Protocol
10. Header Checksum
11. Source Address
12. Destination Address
13. Options
14. Padding
The diagram depicts the fields of an IP packet in the following sequence:
1. Version
2. IHL
3. Service Type
4. Packet Length
5. Identification
6. Flag
7. Fragment Offset
8. Time to Live
9. Protocol
10. Header Checksum
11. Source Address
12. Destination Address
13. Options
14. Padding
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MAC Layer Frame Format
The Layer 2 data link frame usually contains header information with a data link source and destination address, trailer information, and the actual transmitted data. The data link source address is the Layer 2 address of the interface that sent the data link frame. The data link destination address is the Layer 2 address of the interface of the destination device. Both the source and destination data link interfaces are on the same network. As a packet is forwarded from router to router, the Layer 3 source and destination IP addresses will not change; however, the Layer 2 source and destination data link addresses will change. This process will be examined more closely later in this section.
Note: When NAT (Network Address Translation) is used, the destination IP address does change, but this process is of no concern to IP and is a process performed within a company's network. Routing with NAT is discussed in a later course.
The Layer 3 IP packet is encapsulated in the Layer 2 data link frame associated with that interface. In this example, we will show the Layer 2 Ethernet frame. The figure shows the two compatible versions of Ethernet. Below is a list of the fields in an Ethernet frame and a brief description of each one.
- Preamble - Seven bytes of alternating 1s and 0s, used to synchronize signals
- Start-of-frame (SOF) delimiter - 1 byte signaling the beginning of the frame
- Destination address - 6 byte MAC address of the sending device on the local segment
- Source address - 6 byte MAC address of the receiving device on the local segment
- Type/length - 2 bytes specifying either the type of upper layer protocol (Ethernet II frame format) or the length of the data field (IEEE 802.3 frame format)
- Data and pad - 46 to 1500 bytes of data; zeros used to pad any data packet less than 46 bytes
- Frame check sequence (FCS) - 4 bytes used for a cyclical redundancy check to make sure the frame is not corrupted
1.4.1 - Packet Fields and Frame Fields
The diagram depicts the fields and field length in bytes for an Ethernet and 8 0 2 dot 3 frame.
Ethernet frame:
- Preamble (8 bytes)
- Destination Address (6 bytes)
- Source Address (6 bytes)
- Type (2 bytes)
- Data (46-1500)
- FCS (4 bytes)
IEEE 8 0 2 dot 3 frame:
- Preamble (7 bytes)
- S O F (1 byte)
- Destination Address (6 bytes)
- Source Address (6 bytes)
- Length (2 bytes)
- 8 0 2 dot 2 header and data (46-1500)
- FCS (4 bytes)
The diagram depicts the fields and field length in bytes for an Ethernet and 8 0 2 dot 3 frame.
Ethernet frame:
- Preamble (8 bytes)
- Destination Address (6 bytes)
- Source Address (6 bytes)
- Type (2 bytes)
- Data (46-1500)
- FCS (4 bytes)
IEEE 8 0 2 dot 3 frame:
- Preamble (7 bytes)
- S O F (1 byte)
- Destination Address (6 bytes)
- Source Address (6 bytes)
- Length (2 bytes)
- 8 0 2 dot 2 header and data (46-1500)
- FCS (4 bytes)
1.4.2 Best Path and Metric
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Best Path
Determining a router's best path involves the evaluation of multiple paths to the same destination network and selecting the optimum or "shortest" path to reach that network. Whenever multiple paths to reach the same network exist, each path uses a different exit interface on the router to reach that network. The best path is selected by a routing protocol based on the value or metric it uses to determine the distance to reach a network. Some routing protocols, such as RIP, use simple hop-count, which the number of routers between a router and the destination network. Other routing protocols, such as OSPF, determine the shortest path by examining the bandwidth of the links, and using the links with the fastest bandwidth from a router to the destination network.
Dynamic routing protocols typically use their own rules and metrics to build and update routing tables. A metric is the quantitative value used to measure the distance to a given route. The best path to a network is the path with the lowest metric. For example, a router will prefer a path that is 5 hops away over a path that is 10 hops away.
The primary objective of the routing protocol is to determine the best paths for each route to include in the routing table. The routing algorithm generates a value, or a metric, for each path through the network. Metrics can be based on either a single characteristic or several characteristics of a path. Some routing protocols can base route selection on multiple metrics, combining them into a single metric. The smaller the value of the metric, the better the path.
Comparing Hop Count and Bandwidth Metrics
Two metrics that are used by some dynamic routing protocols are:
- Hop count-Hop count is the number of routers that a packet must travel through before reaching its destination. Each router is equal to one hop. A hop count of four indicates that a packet must pass through four routers to reach its destination. If multiple paths are available to a destination, the routing protocol, such as RIP, picks the path with the least number of hops.
- Bandwidth-Bandwidth is the data capacity of a link, sometimes referred to as the speed of the link. For example, Cisco's implementation of the OSPF routing protocol uses bandwidth as its metric. The best path to a network is determined by the path with an accumulation of links that have the highest bandwidth values, or the fastest links. The use of bandwidth in OSPF will be explained in Chapter 11.
When hop count is used as the metric, the resulting path may sometimes be suboptimal. For example, consider the network shown in the figure. If RIP is the routing protocol used by the three routers, then R1 will choose the suboptimal route through R3 to reach PC2 because this path has fewer hops. Bandwidth is not considered. However, if OSPF is used as the routing protocol, then R1 will choose the route based on bandwidth. Packets will be able to reach their destination sooner using the two, faster T1 links as compared to the single, slower 56 Kbps link.
1.4.2 - Best Path and Metric
The animation compares hop count and bandwidth when used as a metric to determine the best route to a destination. The diagram shows the following network topology.
PC1 is connected to router R1. Router R1 is connected to router R2 via a T1 WAN link. Router R2 is connected to router R3 via a T1 WAN link. Router R1 is connected to router R3 via a 56 Kilobit per second WAN link. PC2 is connected to router R3.
As the animation progresses, a packet is sent from PC1 to PC2. The routers use hop count as the metric. The packet travels from R1 to R3 over the 56 Kilobit per second WAN link, which is only one hop but a slow link.
Another packet is sent from PC1 to PC2. This time the routers use bandwidth as the metric. The packet travels from R1 to R2 to R3 over the two T1 links, which are 1.544 megabit per second WAN links. There are two hops, but the links are very fast.
The animation compares hop count and bandwidth when used as a metric to determine the best route to a destination. The diagram shows the following network topology.
PC1 is connected to router R1. Router R1 is connected to router R2 via a T1 WAN link. Router R2 is connected to router R3 via a T1 WAN link. Router R1 is connected to router R3 via a 56 Kilobit per second WAN link. PC2 is connected to router R3.
As the animation progresses, a packet is sent from PC1 to PC2. The routers use hop count as the metric. The packet travels from R1 to R3 over the 56 Kilobit per second WAN link, which is only one hop but a slow link.
Another packet is sent from PC1 to PC2. This time the routers use bandwidth as the metric. The packet travels from R1 to R2 to R3 over the two T1 links, which are 1.544 megabit per second WAN links. There are two hops, but the links are very fast.
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Use the Packet Tracer Activity to determine the best path using routing tables.
Click the Packet Tracer icon for more details.
1.4.2 - Best Path and Metric
Link to Packet Tracer Exploration: Determine Best Path Using Routing Tables.
Use the Packet Tracer Activity to determine the best path using routing tables.
Link to Packet Tracer Exploration: Determine Best Path Using Routing Tables.
Use the Packet Tracer Activity to determine the best path using routing tables.
1.4.3 Equal Cost Load Balancing
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Equal Cost Load Balancing
You may be wondering what happens if a routing table has two or more paths with the same metric to the same destination network. When a router has multiple paths to a destination network and the value of that metric (hop count, bandwidth, etc.) is the same, this is known as an equal cost metric, and the router will perform equal cost load balancing. The routing table will contain the single destination network but will have multiple exit interfaces, one for each equal cost path. The router will forward packets using the multiple exit interfaces listed in the routing table.
If configured correctly, load balancing can increase the effectiveness and performance of the network. Equal cost load balancing can be configured to use both dynamic routing protocols and static routes. Equal cost load balancing is discussed in more detail in Chapter 8, "The Routing Table: A Closer Look".
Equal Cost Paths and Unequal Cost Paths
Just in case you are wondering, a router can send packets over multiple networks even when the metric is not the same if it is using a routing protocol that has this capability. This is known as unequal cost load balancing. EIGRP (as well as IGRP) are the only routing protocols that can be configured for unequal cost load balancing. Unequal cost load balancing in EIGRP is not discussed in this course but is covered in CCNP.
1.4.3 - Equal Cost Load Balancing
The animation depicts equal-cost load balancing used by routers using the following network topology.
Network Topology:
PC1 is connected to router R1. Router R1 is connected to routers R2 and R4 via T1 WAN links. Router R2 is connected to router R3 via a T1 WAN link. Router R4 is connected to router R3 via a T1 WAN link.
As the animation progresses, multiple packets are sent from PC1 to PC2. The routers load balance the packet stream by sending packets across multiple links. Because the links are all the same bandwidth, equal-cost load balancing sends some packets via the R1 to R2 to R3 route and other packets via the R1 to R4 to R3 route.
The animation depicts equal-cost load balancing used by routers using the following network topology.
Network Topology:
PC1 is connected to router R1. Router R1 is connected to routers R2 and R4 via T1 WAN links. Router R2 is connected to router R3 via a T1 WAN link. Router R4 is connected to router R3 via a T1 WAN link.
As the animation progresses, multiple packets are sent from PC1 to PC2. The routers load balance the packet stream by sending packets across multiple links. Because the links are all the same bandwidth, equal-cost load balancing sends some packets via the R1 to R2 to R3 route and other packets via the R1 to R4 to R3 route.
Page 2:
Use the Packet Tracer Activity to explore a routing table that is using equal cost load balancing.
Click the Packet Tracer icon for more details.
1.4.3 - Equal Cost Load Balancing
Link to Packet Tracer Exploration: Equal-Cost Load Balancing
Use the Packet Tracer Activity to explore a routing table that is using equal-cost load balancing.
Link to Packet Tracer Exploration: Equal-Cost Load Balancing
Use the Packet Tracer Activity to explore a routing table that is using equal-cost load balancing.
1.4.4 Path Determination
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Path Determination
Packet forwarding involves two functions:
- Path determination function
- Switching function
One of three path determinations results from this search:
Directly Connected Network - If the destination IP address of the packet belongs to a device on a network that is directly connected to one of the router's interfaces, that packet is forwarded directly to that device. This means that the destination IP address of the packet is a host address on the same network as this router's interface.
Remote Network - If the destination IP address of the packet belongs to a remote network, then the packet is forwarded to another router. Remote networks can only be reached by forwarding packets to another router.
No Route Determined - If the destination IP address of the packet does not belong to either a connected or remote network, and if the router does not have a default route, then the packet is discarded. The router sends an ICMP unreachable message to the source IP address of the packet.
In the first two results, the router re-encapsulates the IP packet into the Layer 2 data link frame format of the exit interface. The type of Layer 2 encapsulation is determined by the type of interface. For example, if the exit interface is FastEthernet, the packet is encapsulated in an Ethernet frame. If the exit interface is a serial interface configured for PPP, the IP packet is encapsulated in a PPP frame.
The following section demonstrates this process.
1.4.4 - Path Determination
The diagram depicts how a router finds the best path using the following network topology.
Network Topology:
PC1 and PC2 are connected to router R1. R1 is connected to several other routers and then to R6. PC3 is connected to R6.
A question is shown over the routers stating, Which path?
The diagram depicts how a router finds the best path using the following network topology.
Network Topology:
PC1 and PC2 are connected to router R1. R1 is connected to several other routers and then to R6. PC3 is connected to R6.
A question is shown over the routers stating, Which path?
1.4.5 Switching Function
Page 1:
Switching Function
After the router has determined the exit interface using the path determination function, the router needs to encapsulate the packet into the data link frame of the outgoing interface.
The switching function is the process used by a router to accept a packet on one interface and forward it out another interface. A key responsibility of the switching function is to encapsulate packets in the appropriate data link frame type for the outgoing data link.
What does a router do with a packet received from one network and destined for another network? The router performs the following three major steps:
1. Decapsulates the Layer 3 packet by removing the Layer 2 frame header and trailer.
2. Examines the destination IP address of the IP packet to find the best path in the routing table.
3. Encapsulates Layer 3 packet into a new Layer 2 frame and forwards the frame out the exit interface.
Click Play to view the animation.
As the Layer 3 IP packet is forwarded from one router to the next, the IP packet remains unchanged, with the exception of the Time To Live (TTL) field. When a router receives an IP packet, it decrements the TTL by one. If the resulting TTL value is zero, the router discards the packet. The TTL is used to prevent IP packets from traveling endlessly over networks due to a routing loop or other misfunction in the network. Routing loops are discussed in a later a chapter.
As the IP packet is decapsulated from one Layer 2 frame and encapsulated into a new Layer 2 frame, the data link destination address and source address will change as the packet is forwarded from one router to the next. The Layer 2 data link source address represents the Layer 2 address of the outbound interface. The Layer 2 destination address represents the Layer 2 address of the next-hop router. If the next hop is the final destination device, it will be the Layer 2 address of that device.
It is very likely that the packet will be encapsulated in a different type of Layer 2 frame than the one in which it was received. For example, the packet might be received by the router on a FastEthernet interface, encapsulated in an Ethernet frame, and forwarded out a serial interface encapsulated in a PPP frame.
Remember, as a packet travels from the source device to the final destination device, the Layer 3 IP addresses do not change. However, the Layer 2 data link addresses change at every hop as the packet is decapsulated and re-encapsulated in a new frame by each router.
1.4.5 - Switching Function
The animation depicts a day in the life of a packet - Step 1. The diagram shows the following network topology.
Network Topology:
PC1 is connected to router R1. Router R1 is connected to router R2 via an Ethernet link. Router R2 is connected to router R3 via a WAN link. PC2 is connected to router R3.
PC1 sends a packet with its own source IP address, 192.168.1.10, to PC2 with a destination address of 192.168.4.10. Router R2 says: "I have a packet for PC2. What path does it need to take? I see PC2 is on a different network. Since PC2 is on different network, I'll encapsulate the packet and send it to the router on my network. Let me find that MAC address."
The animation depicts a day in the life of a packet - Step 1. The diagram shows the following network topology.
Network Topology:
PC1 is connected to router R1. Router R1 is connected to router R2 via an Ethernet link. Router R2 is connected to router R3 via a WAN link. PC2 is connected to router R3.
PC1 sends a packet with its own source IP address, 192.168.1.10, to PC2 with a destination address of 192.168.4.10. Router R2 says: "I have a packet for PC2. What path does it need to take? I see PC2 is on a different network. Since PC2 is on different network, I'll encapsulate the packet and send it to the router on my network. Let me find that MAC address."
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Path Determination and Switching Function Details
Can you describe the exact details of what happens to a packet at Layer 2 and Layer 3 as it travels from source to destination? If not, study the animation and follow along with the discussion until you can describe the process on your own.
Click Play to view the animation.
Step 1: PC1 has a packet to be sent to PC2
PC1 encapsulates the IP packet into an Ethernet frame with the destination MAC address of R1's FastEthernet 0/0 interface.
How does PC1 know to forward to packet to R1 and not directly to PC2? PC1 has determined that the IP source and IP destination addresses are on different networks.
PC1 knows the network it belongs to by doing an AND operation on its own IP address and subnet mask, which results in its network address. PC1 does this same AND operation using the packet destination IP address and the PC1 subnet mask. If the result is the same as its own network, PC1 knows that the destination IP address is on its own network and it does not need to forward the packet to the default gateway, the router. If the AND operation results in a different network address, PC1 knows that the destination IP address is not on its own network and that it must forward this packet to the default gateway, the router.
Note: If an AND operation with the destination IP address of the packet and the subnet mask of PC1 results in a different network address than what PC1 has determined to be its own network address, this address does not necessarily reflect the actual remote network address. PC1 only knows that if the destination IP address is on its own network, the masks will be the same and the network addresses would be the same. The mask of the remote network might be a different mask. If the destination IP address results in a different network address, PC1 will not know the actual remote network address - it only knows that it is not on its own network.
How does PC1 determine the MAC address of the default gateway, router R1? PC1 checks its ARP table for the IP address of the default gateway and its associated MAC address.
What if this entry does not exist in the ARP table? PC1 sends an ARP request and router R1 sends back an ARP reply.
Step 2: Router R1 receives the Ethernet frame
1. Router R1 examines the destination MAC address, which matches the MAC address of the receiving interface, FastEthernet 0/0. R1 will therefore copy the frame into its buffer.
2. R1 sees that the Ethernet Type field is 0x800, which means that the Ethernet frame contains an IP packet in the data portion of the frame.
3. R1 decapsulates the Ethernet frame.
4. Because the destination IP address of the packet does not match any of R1's directly connected networks, the router consults its routing table to route this packet. R1 searches the routing table for a network address and subnet mask that would include this packet's destination IP address as a host address on that network. In this example, the routing table has a route for the 192.168.4.0/24 network. The destination IP address of the packet is 192.168.4.10, which is a host IP address on that network.
R1's route to the 192.168.4.0/24 network has a next-hop IP address of 192.168.2.2 and an exit interface of FastEthernet 0/1. This means that the IP packet will be encapsulated in a new Ethernet frame with the destination MAC address of the next-hop router's IP address. Because the exit interface is on an Ethernet network, R1 must resolve the next-hop IP address with a destination MAC address.
5. R1 looks up the next-hop IP address of 192.168.2.2 in its ARP cache for its FastEthernet 0/1 interface. If the entry is not in the ARP cache, R1 sends an ARP request out its FastEthernet 0/1 interface. R2 sends back an ARP reply. R1 then updates its ARP cache with an entry for 192.168.2.2 and the associated MAC address.
6. The IP packet is now encapsulated into a new Ethernet frame and forwarded out R1's FastEthernet 0/1 interface.
1.4.5 - Switching Function
The animation depicts a day in the life of a packet - Step 2. The diagram uses the network topology described in 1.4.5 diagram 1.
Router R1 receives the frame from PC1. Router R1 says: "Hmm... a frame sent to my MAC address. Let me investigate further. I can see from the type and destination IP address that this packet needs to be forwarded. I have a route out my FA0/1 interface to reach PC2. Let me rebuild the information in the frame. My ARP table tells me that R2 uses MAC addresses 0B to 31. My own FA0/1 MAC address is 00 to 20. The frame is now ready for me to send out my FA0/1 interface."
The animation depicts a day in the life of a packet - Step 2. The diagram uses the network topology described in 1.4.5 diagram 1.
Router R1 receives the frame from PC1. Router R1 says: "Hmm... a frame sent to my MAC address. Let me investigate further. I can see from the type and destination IP address that this packet needs to be forwarded. I have a route out my FA0/1 interface to reach PC2. Let me rebuild the information in the frame. My ARP table tells me that R2 uses MAC addresses 0B to 31. My own FA0/1 MAC address is 00 to 20. The frame is now ready for me to send out my FA0/1 interface."
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Step 3: Packet arrives at router R2
Click Play to view the animation.
1. Router R2 examines the destination MAC address, which matches the MAC address of the receiving interface, FastEthernet 0/0. R1 will therefore copy the frame into its buffer.
2. R2 sees that the Ethernet Type field is 0x800, which means that the Ethernet frame contains an IP packet in the data portion of the frame.
3. R2 decapsulates the Ethernet frame.
4. Because the destination IP address of the packet does not match any of R2's interface addresses, the router consults its routing table to route this packet. R2 searches the routing table for the packet's destination IP address using the same process R1 used.
R2's routing table has a route to the 192.168.4.0/24 route, with a next-hop IP address of 192.168.3.2 and an exit interface of Serial 0/0/0. Because the exit interface is not an Ethernet network, R2 does not have to resolve the next-hop-IP address with a destination MAC address.
When the interface is a point-to-point serial connection, R2 encapsulates the IP packet into the proper data link frame format used by the exit interface (HDLC, PPP, etc.). In this case, the Layer 2 encapsulation is PPP; therefore, the data link destination address is set to a broadcast. Remember, there are no MAC addresses on serial interfaces.
5. The IP packet is now encapsulated into a new data link frame, PPP, and sent out the serial 0/0/0 exit interface.
1.4.5 - Switching Function
The animation depicts a day in the life of a packet - Step 3. The diagram uses the network topology described in 1.4.5 diagram 1.
Router R2 receives the frame from R1. Router R2 says: "Hmm... a frame sent to my MAC address. Let me investigate further. I can see from the type and destination IP address that this packet needs to be forwarded. I have a route on my S0/0/0 interface to reach PC2. Let me rebuild the information in the frame. Because the packet is being sent over a serial connection, I'll use a broadcast destination address. I don't need to put in the source address. R3 knows its coming from me."
The animation depicts a day in the life of a packet - Step 3. The diagram uses the network topology described in 1.4.5 diagram 1.
Router R2 receives the frame from R1. Router R2 says: "Hmm... a frame sent to my MAC address. Let me investigate further. I can see from the type and destination IP address that this packet needs to be forwarded. I have a route on my S0/0/0 interface to reach PC2. Let me rebuild the information in the frame. Because the packet is being sent over a serial connection, I'll use a broadcast destination address. I don't need to put in the source address. R3 knows its coming from me."
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Step 4: The packet arrives at R3
1. R3 receives and copies the data link PPP frame into its buffer.
2. R3 decapsulates the data link PPP frame.
3. R3 searches the routing table for the destination IP address of the packet. The search of the routing table results in a network that is one of R3's directly connected networks. This means that the packet can be sent directly to the destination device and does not need to be sent to another router.
Because the exit interface is a directly connected Ethernet network, R3 needs to resolve the destination IP address of the packet with a destination MAC address.
4. R3 searches for the packet's destination IP address of 192.168.4.10 in its ARP cache. If the entry is not in the ARP cache, R3 sends an ARP request out its FastEthernet 0/0 interface. PC2 sends back an ARP reply with its MAC address. R3 updates its ARP cache with an entry for 192.168.4.10 and the MAC address that was returned in the ARP reply.
5. The IP packet is encapsulated into a new data link, Ethernet frame and sent out R3's FastEthernet 0/0 interface.
Step 5: The Ethernet Frame with encapsulated IP packet arrives at PC2
1. PC2 examines the destination MAC address, which matches the MAC address of the receiving interface, its Ethernet NIC. PC2 will therefore copy the rest of the frame into its buffer.
2. PC2 sees that the Ethernet Type field is 0x800, which means that the Ethernet frame contains an IP packet in the data portion of the frame.
3. PC2 decapsulates the Ethernet frame and passes the IP packet to the IP process of its operating system.
Summary
We have just examined the encapsulation and decapsulation process of a packet as it is forwarded from router to router, from the originating source device the final destination device. We have also been introduced to the routing table lookup process, which will be discussed more thoroughly in a later chapter. We have seen that routers are not involved only in Layer 3 routing decisions, but that they also participate in Layer 2 processes, including encapsulation, and on Ethernet networks, ARP. Routers also participate in Layer 1, which is used to transmit and receive the data bits over the physical medium.
Routing tables contain both directly connected networks and remote networks. It is because routers contain addresses for remote networks in their routing tables that routers know where to send packets destined other networks, including the Internet. In the following chapters will learn how the routers build and maintain these routing tables - either by the use of manually entered static routes or through the use of dynamic routing protocols.
1.4.5 - Switching Function
The animation depicts a day in the life of a packet - Step 4. The diagram uses the network topology described in 1.4.5 diagram 1.
Router R3 receives the frame from R2. Router R3 says: "Hmm... a packet sent to me via a broadcast. Let me investigate further. I can see from the type and destination IP address that this packet needs to be forwarded. I have a route out my FA0/0 interface to reach PC2. Let me rebuild the information in the frame. My ARP table tells me that PC2 uses MAC address 0B to 20. Oh look, a frame has been sent to my MAC address, let me process it. The packet also matches my IP address, so it must be mine."
The animation depicts a day in the life of a packet - Step 4. The diagram uses the network topology described in 1.4.5 diagram 1.
Router R3 receives the frame from R2. Router R3 says: "Hmm... a packet sent to me via a broadcast. Let me investigate further. I can see from the type and destination IP address that this packet needs to be forwarded. I have a route out my FA0/0 interface to reach PC2. Let me rebuild the information in the frame. My ARP table tells me that PC2 uses MAC address 0B to 20. Oh look, a frame has been sent to my MAC address, let me process it. The packet also matches my IP address, so it must be mine."
1.5 Router Configuration Labs
1.5.1 Cabling a Network and Basic Router ConfigurationPage 1:
Complete this lab if you need a solid review of device cabling, establishing a console connection, and command-line interface (CLI) basics. If you are comfortable with these skills, you can substitute Lab 1.5.2 Basic Router Configuration for this lab.
Click the lab icon for more details.
1.5.1 - Cabling a Network and Basic Router Configuration
Link to Hands-on Lab: Cabling a Network and Basic Router Configuration
Complete this lab if you need a solid review of device cabling, establishing a console connection, and command-line interface (C L I) basics. If you are comfortable with these skills, you can substitute Lab 1.5.2 Basic Router Configuration for this lab.
Link to Hands-on Lab: Cabling a Network and Basic Router Configuration
Complete this lab if you need a solid review of device cabling, establishing a console connection, and command-line interface (C L I) basics. If you are comfortable with these skills, you can substitute Lab 1.5.2 Basic Router Configuration for this lab.
Page 2:
Use Packet Tracer Activity 1.5.1 to repeat a simulation of Lab 1.5.1. Remember, however, that Packet Tracer is not a substitute for a hands-on lab experience with real equipment.
A summary of the instructions is provided within the activity. Use the Lab PDF for more details.
Click the Packet Tracer icon for more details.
1.5.1 - Cabling a Network and Basic Router Configuration
Link to Packet Tracer Exploration: Cabling a Network with Routers, Switches, and Hosts.
Use Packet Tracer Activity 1.5.1 to repeat a simulation of Lab 1.5.1.
Link to Packet Tracer Exploration: Cabling a Network with Routers, Switches, and Hosts.
Use Packet Tracer Activity 1.5.1 to repeat a simulation of Lab 1.5.1.
1.5.2 Basic Router Configuration
Page 1:
Complete this lab if you have solid skills in device cabling, establishing a console connection, and command-line interface (CLI) basics. If you need a review of these skills, you can substitute Lab 1.5.1 Cabling a Network and Basic Router Configuration for this lab.
Click the lab icon for more details.
1.5.2 - Basic Router Configuration
Link to Hands-on Lab: Basic Router Configuration
Complete this lab if you have solid skills in device cabling, establishing a console connection, and command-line interface (C L I) basics. If you need a review of these skills, you can substitute Lab 1.5.1 Cabling a Network and Basic Router Configuration for this lab.
Link to Hands-on Lab: Basic Router Configuration
Complete this lab if you have solid skills in device cabling, establishing a console connection, and command-line interface (C L I) basics. If you need a review of these skills, you can substitute Lab 1.5.1 Cabling a Network and Basic Router Configuration for this lab.
Page 2:
Use Packet Tracer Activity 1.5.2 to repeat a simulation of Lab 1.5.2. Remember, however, that Packet Tracer is not a substitute for a hands-on lab experience with real equipment.
A summary of the instructions is provided within the activity. Use the Lab PDF for more details.
Click the Packet Tracer icon for more details.
1.5.2 - Basic Router Configuration
Link to Packet Tracer Exploration: Basic Router Configuration
Use Packet Tracer Activity 1.5.2 to repeat a simulation of Lab 1.5.2.
Link to Packet Tracer Exploration: Basic Router Configuration
Use Packet Tracer Activity 1.5.2 to repeat a simulation of Lab 1.5.2.
1.5.3 Challenge Router Configuration
Page 1:
This lab challenges your subnetting and configuration skills. Given an address space and network requirements, you are expected to design and implement an addressing scheme in a two-router topology.
Click the lab icon for more details.
1.5.3 - Challenge Router Configuration
Link to Hands-on Lab: Challenge Router Configuration
This lab challenges your subnetting and configuration skills. Given an address space and the network requirements, you are expected to design and implement an addressing scheme in a two-router topology.
Link to Hands-on Lab: Challenge Router Configuration
This lab challenges your subnetting and configuration skills. Given an address space and the network requirements, you are expected to design and implement an addressing scheme in a two-router topology.
Page 2:
Use Packet Tracer Activity 1.5.3 to repeat a simulation of Lab 1.5.3. Remember, however, that Packet Tracer is not a substitute for a hands-on lab experience with real equipment.
A summary of the instructions is provided within the activity. Use the Lab PDF for more details.
Click the Packet Tracer icon for more details.
1.5.3 - Challenge Router Configuration
Link to Packet Tracer Exploration: Challenge Router Configuration
Use Packet Tracer Activity 1.5.3 to repeat a simulation of Lab 1.5.3.
Link to Packet Tracer Exploration: Challenge Router Configuration
Use Packet Tracer Activity 1.5.3 to repeat a simulation of Lab 1.5.3.
1.6 Summary
1.6.1 Summary and ReviewPage 1:
Summary
This chapter introduced the router. Routers are computers and include many of the same hardware and software components found in a typical PC, such as CPU, RAM, ROM, and an operating system.
The main purpose of a router is to connect multiple networks and forward packets from one network to the next. This means that a router typically has multiple interfaces. Each interface is a member or host on a different IP network.
The router has a routing table, which is a list of networks known by the router. The routing table includes network addresses for its own interfaces, which are the directly connected networks, as well as network addresses for remote networks. A remote network is a network that can only be reached by forwarding the packet to another router.
Remote networks are added to the routing table in two ways: either by the network administrator manually configuring static routes or by implementing a dynamic routing protocol. Static routes do not have as much overhead as dynamic routing protocols; however, static routes can require more maintenance if the topology is constantly changing or is unstable.
Dynamic routing protocols automatically adjust to changes without any intervention from the network administrator. Dynamic routing protocols require more CPU processing and also use a certain amount of link capacity for routing updates and messages. In many cases, a routing table will contain both static and dynamic routes.
Routers make their primary forwarding decision at Layer 3, the Network layer. However, router interfaces participate in Layers 1, 2, and 3. Layer 3 IP packets are encapsulated into a Layer 2 data link frame and encoded into bits at Layer 1. Router interfaces participate in Layer 2 processes associated with their encapsulation. For example, an Ethernet interface on a router participates in the ARP process like other hosts on that LAN.
In the next chapter, we will examine the configuration of static routes and introduce the IP routing table.
1.6.1 - Summary and Review
In this chapter, you have learned to:
- Identify a router as a computer with an operating system (OS) and hardware designed for the routing process.
- Demonstrate the ability to configure devices and apply addresses.
- Describe the structure of a routing table.
- Describe how a router determines a path and switches packets.
In this chapter, you have learned to:
- Identify a router as a computer with an operating system (OS) and hardware designed for the routing process.
- Demonstrate the ability to configure devices and apply addresses.
- Describe the structure of a routing table.
- Describe how a router determines a path and switches packets.
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1.6.1 - Summary and Review
This is a review and is not a quiz. Questions and answers are provided.
Question 1. Describe the internal and external route hardware components and the purpose of each.
Answer:
Central Processing Unit (CPU) - Executes operating system instructions, such as system initialization, routing functions, and network interface control.
Random-Access Memory (RAM) - Stores the routing table and other data structures that the router needs when forwarding packets.
Read-Only Memory (ROM) - Holds basic diagnostic software used when the router is powered on.
Non-Volatile RAM (NV RAM) - Stores the startup configuration, including IP addresses, routing protocol, and other related information. NV RAM is a portion of the Boot ROM chip.
Flash memory - Stores the operating system (Cisco I O S) and other files.
LAN interfaces such as Ethernet and FastEthernet are used for connecting to different LAN's.
WAN interfaces are used for connecting to a variety of serial links, including T1, DSL, and ISDN.
Question 2. Describe the router bootup process from power on to final configuration.
Answer:
1. Perform POST.
2. Execute Bootstrap Loader.
3. Locate the I O S.
4. Load the I O S.
5. Locate the Startup Configuration file.
6. Execute the Startup Configuration file.
7. If the Startup Configuration file does not exist, enter Setup mode.
Note: If a full I O S image cannot be located, a scaled-down version of the I O S is copied from ROM into RAM. This version of I O S is used to help diagnose any problems and can be used to load a complete version of the I O S into RAM. If the startup configuration file does not exist in NV RAM, the router might search for a TFTP server. If the router detects that it has an active link to another configured router, it sends a broadcast searching for a configuration file across the active link.
Question 3. What important features does a router add to the network?
Answer:
- Determining the best path to send packets
- Forwarding packets toward their destination
Question 4. Describe the steps necessary to apply a basic configuration to a router.
Answer:
- Name the router
- Set passwords
- Configure interfaces
- Configure a banner
- Save changes on a router
- Verify basic configuration and router operations
Question 5. Describe the importance of the routing table. What purposes does it serve?
Answer: A routing table provides the router with the necessary information to carry out its primary function of forwarding packets toward the destination network.
Question 6. What are the three basic ways a router learns about networks?
Answer:
- Connected Routes
- Static Routes
- Dynamic Routes
Question 7. For your current studies, what are the most important fields in the IP header and why are they important?
Answer:
Version - version of IP currently used is IPv4
Time to Live (TTL) - number of routers a packet can traverse before being dropped
Source IP address - 32-bit source IP address
Destination IP address - 32-bit destination IP address
Question 8. Describe the encapsulation and decapsulation process as a packet travels from source to destination.
Answer: The source encapsulates data in a packet with source and destination IP addresses. It then encapsulates the packets into a frame with source and destination MAC addresses and sends the frame out as bits on the wire. The frame is received by the source's gateway, a router, and is decapsulated. If the destination MAC address is the router, the router searches the routing table for an outgoing interface to the destination, encapsulates the packet in the appropriate frame format for the outgoing interface with the new source and destination Layer 2 addresses, and forwards the frame out the interface. This process is repeated at each router along the path until the packet reaches the destination. From source to destination, the Layer 2 addresses change at each hop. However, the source and destination addresses do not change.
Question 9. When you think about the difference between the hardware and software of a PC and a router, what do you see as the strengths and weaknesses of each device? Which device do you think is the more powerful and why?
Answer: Your answer should revolve around the understanding that a router is a single-purpose device and a computer is a multipurpose device.
Question 10. As you study, learn, and use the command-line interface on a Cisco router, do you see a time when you may not need to use the C L I to configure routers and switches? What does your vision of network configuration tasks look like without the C L I?
Answer: Answers will vary. However, C L I is still the dominate way to complete configuration tasks. Like DOS commands, C L I commands will most likely continue to be favored among network administrators as the primary way to complete many configuration or verification tasks, even when a more intuitive and easier-to-use G U I becomes available.
Question 11. If you could design your own routing protocol algorithm to route packets, what would its main features be? How would your protocol decide on the best route? Remember, a computer is going to implement your idea, therefore, be specific.
Answer: Answers will vary. Your description should include a step-by-step process. To see psuedocode for current routing algorithms, search the web for Bellman-Ford and Dijkstra algorithms.
Question 12. Although the Internet Protocol is now considered the only protocol to use for Layer 3 addressing, this was not always the case. Investigate and report on some other Layer 3 protocols that serve the same purpose. What features do they share in common with IP? How are they different?
Answer: A hierarchical structure is common to all Layer 3 addressing protocols. Each one identifies a network portion and a host portion. How each does this is different. For example, Novell's Internet Packet Exchange uses an 80-bit address. The first 32 bits are designated network bits and are determined by the administrator. The remaining 48 bits are the same as the MAC address of the host.
This is a review and is not a quiz. Questions and answers are provided.
Question 1. Describe the internal and external route hardware components and the purpose of each.
Answer:
Central Processing Unit (CPU) - Executes operating system instructions, such as system initialization, routing functions, and network interface control.
Random-Access Memory (RAM) - Stores the routing table and other data structures that the router needs when forwarding packets.
Read-Only Memory (ROM) - Holds basic diagnostic software used when the router is powered on.
Non-Volatile RAM (NV RAM) - Stores the startup configuration, including IP addresses, routing protocol, and other related information. NV RAM is a portion of the Boot ROM chip.
Flash memory - Stores the operating system (Cisco I O S) and other files.
LAN interfaces such as Ethernet and FastEthernet are used for connecting to different LAN's.
WAN interfaces are used for connecting to a variety of serial links, including T1, DSL, and ISDN.
Question 2. Describe the router bootup process from power on to final configuration.
Answer:
1. Perform POST.
2. Execute Bootstrap Loader.
3. Locate the I O S.
4. Load the I O S.
5. Locate the Startup Configuration file.
6. Execute the Startup Configuration file.
7. If the Startup Configuration file does not exist, enter Setup mode.
Note: If a full I O S image cannot be located, a scaled-down version of the I O S is copied from ROM into RAM. This version of I O S is used to help diagnose any problems and can be used to load a complete version of the I O S into RAM. If the startup configuration file does not exist in NV RAM, the router might search for a TFTP server. If the router detects that it has an active link to another configured router, it sends a broadcast searching for a configuration file across the active link.
Question 3. What important features does a router add to the network?
Answer:
- Determining the best path to send packets
- Forwarding packets toward their destination
Question 4. Describe the steps necessary to apply a basic configuration to a router.
Answer:
- Name the router
- Set passwords
- Configure interfaces
- Configure a banner
- Save changes on a router
- Verify basic configuration and router operations
Question 5. Describe the importance of the routing table. What purposes does it serve?
Answer: A routing table provides the router with the necessary information to carry out its primary function of forwarding packets toward the destination network.
Question 6. What are the three basic ways a router learns about networks?
Answer:
- Connected Routes
- Static Routes
- Dynamic Routes
Question 7. For your current studies, what are the most important fields in the IP header and why are they important?
Answer:
Version - version of IP currently used is IPv4
Time to Live (TTL) - number of routers a packet can traverse before being dropped
Source IP address - 32-bit source IP address
Destination IP address - 32-bit destination IP address
Question 8. Describe the encapsulation and decapsulation process as a packet travels from source to destination.
Answer: The source encapsulates data in a packet with source and destination IP addresses. It then encapsulates the packets into a frame with source and destination MAC addresses and sends the frame out as bits on the wire. The frame is received by the source's gateway, a router, and is decapsulated. If the destination MAC address is the router, the router searches the routing table for an outgoing interface to the destination, encapsulates the packet in the appropriate frame format for the outgoing interface with the new source and destination Layer 2 addresses, and forwards the frame out the interface. This process is repeated at each router along the path until the packet reaches the destination. From source to destination, the Layer 2 addresses change at each hop. However, the source and destination addresses do not change.
Question 9. When you think about the difference between the hardware and software of a PC and a router, what do you see as the strengths and weaknesses of each device? Which device do you think is the more powerful and why?
Answer: Your answer should revolve around the understanding that a router is a single-purpose device and a computer is a multipurpose device.
Question 10. As you study, learn, and use the command-line interface on a Cisco router, do you see a time when you may not need to use the C L I to configure routers and switches? What does your vision of network configuration tasks look like without the C L I?
Answer: Answers will vary. However, C L I is still the dominate way to complete configuration tasks. Like DOS commands, C L I commands will most likely continue to be favored among network administrators as the primary way to complete many configuration or verification tasks, even when a more intuitive and easier-to-use G U I becomes available.
Question 11. If you could design your own routing protocol algorithm to route packets, what would its main features be? How would your protocol decide on the best route? Remember, a computer is going to implement your idea, therefore, be specific.
Answer: Answers will vary. Your description should include a step-by-step process. To see psuedocode for current routing algorithms, search the web for Bellman-Ford and Dijkstra algorithms.
Question 12. Although the Internet Protocol is now considered the only protocol to use for Layer 3 addressing, this was not always the case. Investigate and report on some other Layer 3 protocols that serve the same purpose. What features do they share in common with IP? How are they different?
Answer: A hierarchical structure is common to all Layer 3 addressing protocols. Each one identifies a network portion and a host portion. How each does this is different. For example, Novell's Internet Packet Exchange uses an 80-bit address. The first 32 bits are designated network bits and are determined by the administrator. The remaining 48 bits are the same as the MAC address of the host.
Page 3:
The Packet Tracer Skills Integration Challenge Activity for this chapter integrates all the knowledge and skills you acquired in previous courses and the first chapter of this course. In this activity, you build a network from the ground up. Starting with an addressing space and network requirements, you must implement a network design that satisfies the specifications.
Packet Tracer Skills Integration Instructions (PDF)
Click the Packet Tracer icon for more details.
1.6.1 - Summary and Review
Link to Packet Tracer Exploration: Chapter 1 - Packet Tracer Skills Integration Challenge.
The Packet Tracer Skills Integration Challenge Activity for this chapter integrates all the knowledge and skills that you have acquired in previous courses and the first chapter of this course. In this activity, you build a network from the ground up. Starting with an addressing space and network requirements, you must implement a network design that satisfies the specifications.
Link to Packet Tracer Exploration: Chapter 1 - Packet Tracer Skills Integration Challenge.
The Packet Tracer Skills Integration Challenge Activity for this chapter integrates all the knowledge and skills that you have acquired in previous courses and the first chapter of this course. In this activity, you build a network from the ground up. Starting with an addressing space and network requirements, you must implement a network design that satisfies the specifications.
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To Learn More
Create a topology similar to that in 1.4.5.2, with several routers, and a LAN at each end. On one LAN add a client host, and on the other end add a web server. On each LAN include a switch between the computer and the router. Assume that each router has a route to each of the LANs, similar to that in 1.4.5.2.
What happens when the host requests a web page from the web server? Look at all of the processes and protocols involved starting with the user entering a URL such as www.cisco.com. This includes protocols learned in Exploration 1 as well as information learned in this chapter.
See if you can determine each of the processes that happen starting with the client needing to resolve www.cisco.com to an IP address which results in the client having to do an ARP Request for the DNS server. What are all of the protocols and processes involved starting with the DNS request to getting the first packet with http information from the web server.
- How is DNS involved?
- How is ARP involved?
- What affect does TCP have between the client and the server? Is the first packet the web server receives from the client the request for the web page?
- What do the switches do when they receive an Ethernet frame? How do they update their MAC address tables and how do they determine how to forward the frame?
- What do the routers do when they receive an IP packet?
- What is the decapsulation and encapsulation process of each frame received and forwarded by the router?
- Is any ARP processes required by the web server and its default gateway (its router)?
1.6.1 - Summary and Review
The diagram depicts a collage of people using computers and networks.
The diagram depicts a collage of people using computers and networks.
1.7 Chapter Quiz
1.7.1 Chapter QuizPage 1:
1.7.1 - Chapter Quiz
1. Which description correctly matches a router component with its function?
a. FLASH - stores the bootstrap program
b. ROM - stores the startup configuration file
c. NV RAM - stores the operating system image
d. RAM - stores the routing tables and ARP cache
2. Construct the sequence of events that occurs during the router startup process by matching one of the four step numbers to the event. (Not all options apply.)
Event:
a. Running-configuration is loaded from NV RAM.
b. Bootstrap loader executes from ROM.
c. Configuration file is loaded from NV RAM.
d. Operating system is found and loaded.
e. Power-on self test is conducted.
f. Setup mode executes after configuration file is loaded.
Step numbers:
Step 1
Step 2
Step 3
Step 4
3. Which two commands can a technician use to determine whether router serial ports have IP addresses that are assigned to them? (Choose two.)
a. show interfaces
b. show interfaces i p brief
c. show controllers all
d. show i p config
e. show i p interface brief
4. Which command sets the privileged mode password to "quiz"?
a. LAB_A(config)#enable secret quiz
b. LAB_A(config)#password secret quiz
c. LAB_A(config)#enable password secret quiz
d. LAB_A(config)#enable secret password quiz
5. Which routing principle is correct?
a. If one router has certain information in its routing table, all adjacent routers have the same information.
b. Routing information about a path from one network to another implies routing information about the reverse, or return path.
c. Every router makes its routing decisions alone, based on the information it has in its own routing table.
d. Every router makes its routing decisions based on the information it has in its own routing table and its neighbor routing tables.
6. What two tasks do dynamic routing protocols perform? (Choose two.)
a. Discover hosts
b. Update and maintain routing tables
c. Propagate host default gateways
d. Network discovery
e. Assign IP addressing
7. A network engineer is configuring a new router. The interfaces have been configured with IP addresses but no routing protocols or static routes have been configured yet. What routes are present in the routing table?
a. Default routes
b. Broadcast routers
c. Direct connections
d. No routes. The routing table is empty.
8. What two statements are correct regarding how a router forwards packets? (Choose two.)
a. If the packet is destined for a remote network, the router forwards the packet out all interfaces that might be a next hop to that network.
b. If the packet is destined for a directly connected network, the router forwards the packet out the exit interface indicated by the routing table.
c. If the packet is destined for a remote network, the router forwards the packet based on the information in the router host table.
d. If the packet is destined for a remote network, the router sends the packet to the next hop IP in the routing table.
e. If the packet is destined for a directly connected network, the router forwards the packet based on the destination MAC address.
f. If the packet is destined for a directly connected network, the router forwards the packet to the switch on the next hop V LAN.
9. Which statement is true regarding metrics used by routing protocols?
a. A metric is the quantitative value that a routing protocol uses to measure a given route.
b. A metric is a Cisco proprietary means to convert distances to a standard unit.
c. Metrics represent a composite value of the amount of packet loss occurring for all routing protocols.
d. Metrics are used by the router to determine if a packet has an error and should be dropped.
e. Metrics are only used by dynamic routing protocols.
10. Refer to the following diagram description to answer the question. A photograph of the back of a router is shown with the following interfaces identified and labeled with numbers:
a. Interface number 1 is Smart Serial 0
b. Interface number 2 is 10/100 Ethernet0/1
c. Interface number 3 is 10/100 Ethernet0/0
d. Interface number 4 is DB-60 Serial
e. Interface number 5 is Console
f. Interface number 6 is Aux
Which port is used to connect a router to a LAN switch?
a. Interface number 4
b. Interface number 5
c. Interface number 6
d. Interface numbers 1 or 4
e. Interface numbers 2 or 3
f. Interface numbers 5 or 6
11. The network administrator configured the i p router 0.0.0.0 0.0.0.0 serial 0/0 command on the router. How does this command appear in the routing table, assuming that the serial0/0 interface is up?
a. D 0.0.0.0/0 is directly connected, Serial0/0
b. S* 0.0.0.0/0 is directly connected, Serial0/0
c. S* 0.0.0.0/0 [1/0] via 192.168.2.2
d. C 0.0.0.0/0 [1/0] via 192.168.2.2
1. Which description correctly matches a router component with its function?
a. FLASH - stores the bootstrap program
b. ROM - stores the startup configuration file
c. NV RAM - stores the operating system image
d. RAM - stores the routing tables and ARP cache
2. Construct the sequence of events that occurs during the router startup process by matching one of the four step numbers to the event. (Not all options apply.)
Event:
a. Running-configuration is loaded from NV RAM.
b. Bootstrap loader executes from ROM.
c. Configuration file is loaded from NV RAM.
d. Operating system is found and loaded.
e. Power-on self test is conducted.
f. Setup mode executes after configuration file is loaded.
Step numbers:
Step 1
Step 2
Step 3
Step 4
3. Which two commands can a technician use to determine whether router serial ports have IP addresses that are assigned to them? (Choose two.)
a. show interfaces
b. show interfaces i p brief
c. show controllers all
d. show i p config
e. show i p interface brief
4. Which command sets the privileged mode password to "quiz"?
a. LAB_A(config)#enable secret quiz
b. LAB_A(config)#password secret quiz
c. LAB_A(config)#enable password secret quiz
d. LAB_A(config)#enable secret password quiz
5. Which routing principle is correct?
a. If one router has certain information in its routing table, all adjacent routers have the same information.
b. Routing information about a path from one network to another implies routing information about the reverse, or return path.
c. Every router makes its routing decisions alone, based on the information it has in its own routing table.
d. Every router makes its routing decisions based on the information it has in its own routing table and its neighbor routing tables.
6. What two tasks do dynamic routing protocols perform? (Choose two.)
a. Discover hosts
b. Update and maintain routing tables
c. Propagate host default gateways
d. Network discovery
e. Assign IP addressing
7. A network engineer is configuring a new router. The interfaces have been configured with IP addresses but no routing protocols or static routes have been configured yet. What routes are present in the routing table?
a. Default routes
b. Broadcast routers
c. Direct connections
d. No routes. The routing table is empty.
8. What two statements are correct regarding how a router forwards packets? (Choose two.)
a. If the packet is destined for a remote network, the router forwards the packet out all interfaces that might be a next hop to that network.
b. If the packet is destined for a directly connected network, the router forwards the packet out the exit interface indicated by the routing table.
c. If the packet is destined for a remote network, the router forwards the packet based on the information in the router host table.
d. If the packet is destined for a remote network, the router sends the packet to the next hop IP in the routing table.
e. If the packet is destined for a directly connected network, the router forwards the packet based on the destination MAC address.
f. If the packet is destined for a directly connected network, the router forwards the packet to the switch on the next hop V LAN.
9. Which statement is true regarding metrics used by routing protocols?
a. A metric is the quantitative value that a routing protocol uses to measure a given route.
b. A metric is a Cisco proprietary means to convert distances to a standard unit.
c. Metrics represent a composite value of the amount of packet loss occurring for all routing protocols.
d. Metrics are used by the router to determine if a packet has an error and should be dropped.
e. Metrics are only used by dynamic routing protocols.
10. Refer to the following diagram description to answer the question. A photograph of the back of a router is shown with the following interfaces identified and labeled with numbers:
a. Interface number 1 is Smart Serial 0
b. Interface number 2 is 10/100 Ethernet0/1
c. Interface number 3 is 10/100 Ethernet0/0
d. Interface number 4 is DB-60 Serial
e. Interface number 5 is Console
f. Interface number 6 is Aux
Which port is used to connect a router to a LAN switch?
a. Interface number 4
b. Interface number 5
c. Interface number 6
d. Interface numbers 1 or 4
e. Interface numbers 2 or 3
f. Interface numbers 5 or 6
11. The network administrator configured the i p router 0.0.0.0 0.0.0.0 serial 0/0 command on the router. How does this command appear in the routing table, assuming that the serial0/0 interface is up?
a. D 0.0.0.0/0 is directly connected, Serial0/0
b. S* 0.0.0.0/0 is directly connected, Serial0/0
c. S* 0.0.0.0/0 [1/0] via 192.168.2.2
d. C 0.0.0.0/0 [1/0] via 192.168.2.2
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