3 Introduction to Dynamic Routing Protocols

3.0 Chapter Introduction

3.0.1 Chapter Introduction

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The data networks that we use in our everyday lives to learn, play, and work range from small, local networks to large, global internetworks. At home, you may have a router and two or more computers. At work, your organization may have multiple routers and switches servicing the data communication needs of hundreds or even thousands of PCs.

In the previous chapters you discovered how routers are used in packet forwarding and that routers learn about remote networks using both static routes and dynamic routing protocols. You also know how routes to remote networks can be configured manually using static routes.

This chapter introduces dynamic routing protocols, including how different routing protocols are classified, what metrics they use to determine best path, and the benefits of using a dynamic routing protocol.

Dynamic routing protocols are usually used in larger networks to ease the administrative and operational overhead of using only static routes. Typically, a network uses a combination of both a dynamic routing protocol and static routes. In most networks, a single dynamic routing protocol is used, however there are cases where different parts of the network may use different routing protocols.

Since the early 1980's, several different dynamic routing protocols have emerged. In this chapter we will begin to discuss some of the characteristics and differences in these routing protocols, however this will become more evident in later chapters when we discuss several of these routing protocols in detail.

Although many networks will only use a single routing protocol or use only static routes, it is important for a network professional to understand the concepts and operations of all the different routing protocols. A network professional must be able to make an informed decision regarding when to use a dynamic routing protocol and which routing protocol is the best choice for a particular environment.

3.0.1 - Chapter Introduction
The diagram depicts images representing static and dynamic routing.
Dynamic routing scales to larger networks.

In this chapter, you will learn to:
- Describe the role of dynamic routing protocols and place these protocols in the context of modern network design.
- Identify several ways to classify routing protocols.
- Describe how metrics are used by routing protocols, and identify the metric types used by dynamic routing protocols.
- Determine the administrative distance of a route and describe its importance in the routing process.
- Identify the different elements in the routing table.
- Given realistic constraints, devise and apply subnetting schemes.

3.1 Introduction and Advantages

3.1.1 Perspective and Background

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The Evolution of Dynamic Routing Protocols

Dynamic routing protocols have been used in networks since the early 1980s. The first version of RIP was released in 1982, but some of the basic algorithms within the protocol were used on the ARPANET as early as 1969.

As networks have evolved and become more complex, new routing protocols have emerged. The figure shows the classification of routing protocols.

One of the earliest routing protocols was Routing Information Protocol (RIP). RIP has evolved into a newer version RIPv2. However, the newer version of RIP still does not scale to larger network implementations. To address the needs of larger networks, two advanced routing protocols were developed: Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS). Cisco developed Interior Gateway Routing Protocol (IGRP) and Enhanced IGRP (EIGRP), which also scales well in larger network implementations.

Additionally, there was the need to interconnect different internetworks and provide routing among them. Border Gateway Routing (BGP) protocol is now used between ISPs as well as between ISPs and their larger private clients to exchange routing information.

With the advent of numerous consumer devices using IP, the IPv4 addressing space is nearly exhausted. Thus IPv6 has emerged. To support the communication based on IPv6, newer versions of the IP routing protocols have been developed (see the IPv6 row in the table).

Note: This chapter presents an overview of the different dynamic routing protocols. More details about RIP, EIGRP, and OSPF routing protocols will be discussed in later chapters. The IS-IS and BGP routing protocols are explained in the CCNP curriculum. IGRP is the predecessor to EIGRP and is now obsolete.

3.1.1 - Perspective and Background
The diagram depicts the evolution and classification of dynamic routing protocols.

The following timeline is presented with the routing protocol and the year in which each was introduced.
EGP - 1982
IGRP - 1985
RIP v1 - 1988
I S-I S - 1990
OSPF v2 - 1991
EIGRP - 1992
RIP v2 - 1994
BGP - 1995
RIP n g - 1997
BGP v6 and OSPF v3 - 1999
I S-I S v6 - 2000

A table is presented that classifies routing protocols as follows:

Interior gateway protocols include distance-vector routing protocols and link-state routing protocols.

Distance vector routing protocols include classful protocols such as RIP and classless protocols such as IGRP, RIP v2, and EIGRP. IPv6 distance vector routing protocols include RIP n g and EIGRP for IPv6.

Link-state routing protocols include classless protocols such as OSPF v2 and I S-I S. IPv6 link state routing protocols include OSPF v3 and I S-I S for IPv6.

Exterior gateway protocols include path vector protocols such as classful EGP and classless BGP v4. IPv6 exterior gateway protocols include BGP v4 for IPv6.

The routing protocols covered in this course are highlighted and include RIP, RIP v2, EIGRP, and OSPF v2.

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The Role of Dynamic Routing Protocol

What exactly are dynamic routing protocols? Routing protocols are used to facilitate the exchange of routing information between routers. Routing protocols allow routers to dynamically share information about remote networks and automatically add this information to their own routing tables. This is shown in the animation.

Routing protocols determine the best path to each network which is then added to the routing table. One of the primary benefits to using a dynamic routing protocol is that routers exchange routing information whenever there is a topology change. This exchange allows routers to automatically learn about new networks and also to find alternate paths when there is a link failure to a current network.

Compared to static routing, dynamic routing protocols require less administrative overhead. However, the expense of using dynamic routing protocols is dedicating part of a router's resources for protocol operation including CPU time and network link bandwidth. Despite the benefits of dynamic routing, static routing still has its place. There are times when static routing is more appropriate and other times when dynamic routing is the better choice. More often than not, you will find a combination of both types of routing in any network that has a moderate level of complexity. We will discuss the advantages and disadvantages of static and dynamic routing later in this chapter.

3.1.1 - Perspective and Background
The animation depicts the role of dynamic routing protocol. Three routers are shown interconnected by WAN links and dynamically passing updates to each other.

3.1.2 Network discovery and routing table maintenance

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The Purpose of Dynamic Routing Protocols

A routing protocol is a set of processes, algorithms, and messages that are used to exchange routing information and populate the routing table with the routing protocol's choice of best paths. The purpose of a routing protocol includes:

Discovery of remote networks
Maintaining up-to-date routing information
Choosing the best path to destination networks
Ability to find a new best path if the current path is no longer available

What are the components of a routing protocol?

Data structures - Some routing protocols use tables and/or databases for its operations. This information is kept in RAM.
Algorithm - An algorithm is a finite list of steps used in accomplishing a task. Routing protocols use algorithms for facilitating routing information and for best path determination.
Routing protocol messages - Routing protocols use various types of messages to discover neighboring routers, exchange routing information, and other tasks to learn and maintain accurate information about the network.

Dynamic Routing Protocol Operation

All routing protocols have the same purpose - to learn about remote networks and to quickly adapt whenever there is a change in the topology. The method that a routing protocol uses to accomplish this depends upon the algorithm it uses and the operational characteristics of that protocol. The operations of a dynamic routing protocol vary depending upon the type of routing protocol and the routing protocol itself. In general, the operations of a dynamic routing protocol can be described as follows:

The router sends and receives routing messages on its interfaces.
The router shares routing messages and routing information with other routers that are using the same routing protocol.
Routers exchange routing information to learn about remote networks.
When a router detects a topology change the routing protocol can advertise this change to other routers.

Play the animation to see dynamic routing protocols in operation.

Note: Understanding dynamic routing protocol operation and concepts and using them in real networks requires a solid knowledge of IP addressing and subnetting. Three subnetting scenarios are available at the end of this chapter for your practice.

3.1.2 - Network discovery and routing table maintenance
The animation depicts the dynamic routing protocol operation. Routing protocols are used to exchange routing information between the routers. The animation is based on the following network topology.

Network Topology:
Router R1 is connected to router R2 via a WAN link with a network address of 172.16.2.0/24. The router R1 interface FA0/0 is connected to LAN 172.16.1.0/24. The router R2 interface FA0/0 is connected to LAN 172.16.3.0/24.

As the animation progresses, the R1 routing protocol consults its routing table and sends an update to R2. Router R2 then consults its routing table and sends an update to R1.

3.1.3 Advantages

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Static Routing Usage

Before identifying the benefits of dynamic routing protocols, we need to consider the reasons why we would use static routing. Dynamic routing certainly has several advantages over static routing. However, static routing is still used in networks today. In fact, networks typically use a combination of both static and dynamic routing.

Static routing has several primary uses, including:

Providing ease of routing table maintenance in smaller networks that are not expected to grow significantly.
Routing to and from stub networks (see Chapter 2).
Use of a single default route, used to represent a path to any network that does not have a more specific match with another route in the routing table.

Static Routing Advantages and Disadvantages

In the table dynamic and static routing features are directly compared. From this comparison, we can list the advantages of each routing method. The advantages of one method are the disadvantages of the other.

Static routing advantages:

Minimal CPU processing.
Easier for administrator to understand.
Easy to configure.

Static routing disadvantages:

Configuration and maintenance is time-consuming.
Configuration is error-prone, especially in large networks.
Administrator intervention is required to maintain changing route information.
Does not scale well with growing networks; maintenance becomes cumbersome.
Requires complete knowledge of the whole network for proper implementation.

Dynamic Routing Advantages and Disadvantages

Dynamic routing advantages:

Administrator has less work maintaining the configuration when adding or deleting networks.
Protocols automatically react to the topology changes.
Configuration is less error-prone.
More scalable, growing the network usually does not present a problem.

Dynamic routing disadvantages:

Router resources are used (CPU cycles, memory and link bandwidth).
More administrator knowledge is required for configuration, verification, and troubleshooting.

3.1.3 - Advantages
The diagram depicts static and dynamic routing advantages and disadvantages. In the table, dynamic and static routing features are directly compared.

Configuration Complexity:
Dynamic routing: Generally independent of the network size.
Static routing: Increases with network size.

Required administrator knowledge:
Dynamic routing: Advanced knowledge required.
Static routing: No extra knowledge required.

Topology changes:
Dynamic routing: Automatically adapts to topology changes.
Static routing: Administrator intervention required.

Scaling:
Dynamic routing: Suitable for simple and complex topologies.
Static routing: Suitable for simple topologies.

Security:
Dynamic routing: Less secure.
Static routing: More secure.

Resource usage:
Dynamic routing: Uses CPU, memory, link bandwidth.
Static routing: No extra resources needed.

Predictability:
Dynamic routing: Route depends on the current topology.
Static routing: Route to destination is always the same.

3.2 Classifying Dynamic Routing Protocols

3.2.1 Overview

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Dynamic Routing Protocols Classification

Routing protocols can be classified into different groups according to their characteristics. The most commonly used routing protocols are:

RIP - A distance vector interior routing protocol
IGRP - The distance vector interior routing developed by Cisco (deprecated from 12.2 IOS and later)
OSPF - A link-state interior routing protocol
IS-IS - A link-state interior routing protocol
EIGRP - The advanced distance vector interior routing protocol developed by Cisco
BGP - A path vector exterior routing protocol

Note: IS-IS and BGP are beyond the scope of this course and are covered in the CCNP curriculum.

The classification criteria are explained later in this chapter.

Drag and drop each protocol onto the correct category in the figure.

3.2.1 - Overview
The diagram depicts an activity in which you must classify dynamic routing protocols from the list of protocols provided.

List of protocols to classify include:
RIP v1
I S-I S
IGRP
BGP
OSPF
EIGRP
RIP v2

Interior Gateway Protocols:
Name four distance vector protocols.

Name two link-state protocols.

Name one exterior gateway protocol.

3.2.2 IGP and EGP

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An autonomous system (AS) - otherwise known as a routing domain - is a collection of routers under a common administration. Typical examples are a company's internal network and an Internet service provider's network. Because the Internet is based on the autonomous system concept, two types of routing protocols are required: interior and exterior routing protocols. These protocols are:

Interior Gateway Protocols (IGP) are used for intra-autonomous system routing - routing inside an autonomous system.
Exterior Gateway Protocols (EGP) are used for inter-autonomous system routing - routing between autonomous systems.

The figure is a simplified view of the difference between IGPs and EGPs. The autonomous system concept will be explained in more detail later in the chapter.

Characteristics of IGP and EGP Routing Protocols

IGPs are used for routing within a routing domain, those networks within the control of a single organization. An autonomous system is commonly comprised of many individual networks belonging to companies, schools, and other institutions. An IGP is used to route within the autonomous system, and also used to route within the individual networks themselves. For example, CENIC operates an autonomous system comprised of California schools, colleges and universities. CENIC uses an IGP to route within its autonomous system in order to interconnect all of these institutions. Each of the educational institutions also uses an IGP of their own choosing to route within its own individual network. The IGP used by each entity provides best path determination within its own routing domains, just as the IGP used by CENIC provides best path routes within the autonomous system itself. IGPs for IP include RIP, IGRP, EIGRP, OSPF, and IS-IS.

Routing protocols, and more specifically the algorithm used by that routing protocol, use a metric to determine the best path to a network. The metric used by the routing protocol RIP is hop count, which is the number of routers that a packet must traverse in reaching another network. OSPF uses bandwidth to determine the shortest path.

EGPs on the other hand, are designed for use between different autonomous systems that are under the control of different administrations. BGP is the only currently-viable EGP and is the routing protocol used by the Internet. BGP is a path vector protocol that can use many different attributes to measure routes. At the ISP level, there are often more important issues than just choosing the fastest path. BGP is typically used between ISPs and sometimes between a company and an ISP. BGP is not part of this course or CCNA; it is covered in CCNP.

3.2.2 - IGP and EGP
The diagram depicts using Interior Gateway Protocols (IGP) and Exterior Gateway Protocols (EGP). IGP's are used for intra-autonomous system routing or routing inside an autonomous system. EGP's are used for inter-autonomous system routing or routing between autonomous systems. The diagram is based on the following network topology.

Network Topology:
Two groups of five routers are shown. Group 1 routers are all interconnected within a cloud labeled autonomous system 100. Group 2 routers are all interconnected within a cloud labeled autonomous system 200. A Group 1 edge router is connected to a Group 2 edge router. An exterior gateway protocol such as BGP exchanges routes between the Group 1 and Group 2 edge routers. Interior Gateway Protocols exchange routes between the router within Group 1 or Group 2. These include RIP, IGRP, EIGRP, OSPF, and I S-I S.

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In this activity, the network has already been configured within the autonomous systems. You will configure a default route from AS2 and AS3 (two different companies) to the ISP (AS1) to simulate the Exterior Gateway Routing that would take place from both companies to their ISP. Then you will configure a static route from the ISP (AS1) to AS2 and AS3 to simulate the Exterior Gateway Routing that would take place from the ISP to its 2 customers AS2 and AS3. View the routing table before and after both static routes and default routes are added to observe how the routing table has changed.

Click the Packet Tracer icon for more details.

3.2.2 - IGP and EGP
Link to Packet Tracer Exploration: Characteristics of IGP and EGP Routing Protocols

In this activity, the network has already been configured within the autonomous systems. You configure a default route from A S2 and A S3 (two different companies) to the ISP (A S1) to simulate the Exterior Gateway Routing that takes place from both companies to their ISP. Then you configure a static route from the ISP (A S1) to A S2 and A S3 to simulate the Exterior Gateway Routing that would take place from the ISP to its two customers A S2 and A S3. View the routing table before and after both static routes and default routes are added to observe how the routing table has changed.

3.2.3 Distance Vector and Link State

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Interior Gateway Protocols (IGPs) can be classified as two types:

Distance vector routing protocols
Link-state routing protocols

Distance Vector Routing Protocol Operation

Distance vector means that routes are advertised as vectors of distance and direction. Distance is defined in terms of a metric such as hop count and direction is simply the next-hop router or exit interface. Distance vector protocols typically use the Bellman-Ford algorithm for the best path route determination.

Some distance vector protocols periodically send complete routing tables to all connected neighbors. In large networks, these routing updates can become enormous, causing significant traffic on the links.

Play the animation to see the operation of distance vector routing protocols.

Although the Bellman-Ford algorithm eventually accumulates enough knowledge to maintain a database of reachable networks, the algorithm does not allow a router to know the exact topology of an internetwork. The router only knows the routing information received from its neighbors.

Distance vector protocols use routers as sign posts along the path to the final destination. The only information a router knows about a remote network is the distance or metric to reach that network and which path or interface to use to get there. Distance vector routing protocols do not have an actual map of the network topology.

Distance vector protocols work best in situations where:

The network is simple and flat and does not require a special hierarchical design.
The administrators do not have enough knowledge to configure and troubleshoot link-state protocols.
Specific types of networks, such as hub-and-spoke networks, are being implemented.
Worst-case convergence times in a network are not a concern.

Distance vector routing protocol functions and operations will be explained in the next chapter. You will also learn about the operations and configuration of the distance vector routing protocols RIP and EIGRP.

3.2.3 - Distance Vector and Link State
The animation depicts distance vector protocol operation between routers. Each router has a routing table and is running a distance vector routing protocol. Distance vector routing protocols periodically pass the entire routing table. The animation is based on the following network topology.

Network Topology:
Four routers, R1, R2, R3, and R4, are interconnected. Routers R1, R3, and R4 all connect to router R2 via WAN links.

As the animation progresses, a route is received by R1 from another router outside the four routers in the topology. R1 updates its routing table and sends the entire routing table as an update to R2. R2 updates its routing table and sends its entire routing table as an update to R3 and R4.

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Link-state Protocol Operation

In contrast to distance vector routing protocol operation, a router configured with a link-state routing protocol can create a "complete view" or topology of the network by gathering information from all of the other routers. To continue our analogy of sign posts, using a link-state routing protocol is like having a complete map of the network topology. The sign posts along the way from source to destination are not necessary, because all link-state routers are using an identical "map" of the network. A link-state router uses the link-state information to create a topology map and to select the best path to all destination networks in the topology.

Play the animation.

With some distance vector routing protocols, routers send periodic updates of their routing information to their neighbors. Link-state routing protocols do not use periodic updates. After the network has converged, a link-state update only sent when there is a change in the topology. For example, the link-state update in the animation is not sent until the 172.16.3.0 network goes down.

Link-state protocols work best in situations where:

The network design is hierarchical, usually occurring in large networks.
The administrators have a good knowledge of the implemented link-state routing protocol.
Fast convergence of the network is crucial.

Link-state routing protocol functions and operations will be explained in later chapters. You will also learn about the operations and configuration of the link-state routing protocol OSPF.

3.2.3 - Distance Vector and Link State
The animation depicts link-state protocol operation between routers. Each router has a link-state database and is running a link-state routing protocol. Link-state protocols pass updates when a link's state changes. The animation is based on the same network topology as 3.2.3 diagram 1.

As the animation progresses, a local network on R1 goes down. R1 updates its routing table and sends a link update to R2. R2 updates its routing table and sends link updates to R3 and R4.

3.2.4 Classful and Classless

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Classful Routing Protocols

Classful routing protocols do not send subnet mask information in routing updates. The first routing protocols such as RIP, were classful. This was at a time when network addresses were allocated based on classes, class A, B, or C. A routing protocol did not need to include the subnet mask in the routing update because the network mask could be determined based on the first octet of the network address.

Classful routing protocols can still be used in some of today's networks, but because they do not include the subnet mask they cannot be used in all situations. Classful routing protocols cannot be used when a network is subnetted using more than one subnet mask, in other words classful routing protocols do not support variable length subnet masks (VLSM).

There are other limitations to classful routing protocols including their inability to support discontiguous networks. Classful routing protocols, discontiguous networks and VLSM will all be discussed in later chapters.

Classful routing protocols include RIPv1 and IGRP.

Classless Routing Protocols

Classless routing protocols include the subnet mask with the network address in routing updates. Today's networks are no longer allocated based on classes and the subnet mask cannot be determined by the value of the first octet. Classless routing protocols are required in most networks today because of their support for VLSM, discontiguous networks and other features which will be discussed in later chapters.

In the figure, notice that the classless version of the network is using both /30 and /27 subnet masks in the same topology. Also notice that this topology is using a discontiguous design.

Classless routing protocols are RIPv2, EIGRP, OSPF, IS-IS, BGP.

3.2.4 - Classful and Classless
The diagram depicts classful versus classless routing. The diagram is based on the following network topology.

Network Topology:
Three routers, R1, R2, and R3, are interconnected in a full mesh by three WAN links. Each router has a LAN attached to it. The same topology is used for classful and classless addressing.

Classful Routing:
With classful addressing, the subnet mask is the same throughout the topology. In the classful example, the network addressing appears as follows:
R1 to R2 WAN link network and subnet mask: 172.16.2.0/24
R2 to R3 WAN link network and subnet mask: 172.16.6.0/24
R3 to R1 WAN link network and subnet mask: 172.16.4.0/24
R1 LAN network and subnet mask: 172.16.3.0/24
R2 LAN network and subnet mask: 172.16.1.0/24
R3 LAN network and subnet mask: 172.16.5.0/24

Classless Routing:
With classless addressing, the subnet mask can vary in the topology. In the classless example, the network addressing appears as follows:
R1 to R2 WAN link network and subnet mask: 192.168.1.0/30
R2 to R3 WAN link network and subnet mask: 192.168.1.4/30
R3 to R1 WAN link network and subnet mask: 192.168.1.8/30
R1 LAN network and subnet mask: 172.16.1.32/27
R2 LAN network and subnet mask: 172.16.1.64/27
R3 LAN network and subnet mask: 172.16.1.96/27

3.2.5 Convergence

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What is Convergence?

Convergence is when all routers' routing tables are at a state of consistency. The network has converged when all routers have complete and accurate information about the network. Convergence time is the time it takes routers to share information, calculate best paths, and update their routing tables. A network is not completely operable until the network has converged; therefore, most networks require short convergence times.

Convergence is both collaborative and independent. The routers share information with each other but must independently calculate the impacts of the topology change on their own routes. Because they develop an agreement with the new topology independently, they are said to converge on this consensus.

Convergence properties include the speed of propagation of routing information and the calculation of optimal paths. Routing protocols can be rated based on the speed to convergence; the faster the convergence, the better the routing protocol. Generally, RIP and IGRP are slow to converge, whereas EIGRP and OSPF are faster to converge.

3.2.5 - Convergence
The diagram depicts comparing the convergence speed of various routing protocols. RIP and IGRP generally provide slower convergence. EIGRP and OSPF generally provide faster convergence. The diagram is based on the following network topology.

Network Topology:
Three routers, R1, R2, and R3, are interconnected by three WAN links. Each router has a LAN attached to it. The LAN link connected to R2 is down.

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In this activity, the network has already been configured with 2 routers, 2 switches and 2 hosts. A new LAN will be added and you will watch the network converge.

Click the Packet Tracer icon for more details.

3.2.5 - Convergence
Link to Packet Tracer Exploration: Convergence

In this activity, the network has already been configured with two routers, two switches, and two hosts. A new LAN will be added, and you will watch the network converge.

3.3 Metrics

3.3.1 Purpose of a Metric

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There are cases when a routing protocol learns of more than one route to the same destination. To select the best path, the routing protocol must be able to evaluate and differentiate between the available paths. For this purpose a metric is used. A metric is a value used by routing protocols to assign costs to reach remote networks. The metric is used to determine which path is most preferable when there are multiple paths to the same remote network.

Each routing protocol uses its own metric. For example, RIP uses hop count, EIGRP uses a combination of bandwidth and delay, and Cisco's implementation of OSPF uses bandwidth. Hop count is the easiest metric to envision. The hop count refers to the number of routers a packet must cross to reach the destination network. For R3 in the figure, network 172.16.3.0 is two hops, or two routers away.

Note: The metrics for a particular routing protocol and how they are calculated will be discussed in the chapter for that routing protocol.

3.3.1 - Purpose of a Metric
The diagram depicts routing metrics using hop count. The diagram is based on the following network topology.

Network Topology:
The network consists of three routers, R1, R2, and R3. Router R1 is connected to router R2 via a WAN link. Router R2 is connected to router R3 via a WAN link. The router R1 interface FA0/0 is connected to LAN 172.16.3.0/24.

Routing tables from each of the three routers are shown displaying the metric to the R1 LAN 172.16.3.0/24.

Router R1:
Net: 172.16.3.0
Hops: 0

Router R2:
Net: 172.16.3.0
Hops: 1

Router R3:
Net: 172.16.3.0
Hops: 2

3.3.2 Metrics and Routing Protocols

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The Metric Parameters

Different routing protocols use different metrics. The metric used by one routing protocol is not comparable to the metric used by another routing protocol. Two different routing protocols might choose different paths to the same destination due to using different metrics.

Play the animation.

RIP would choose the path with the least amount of hops, whereas OSPF would choose the path with the highest bandwidth.

Metrics used in IP routing protocols include:

Hop count - A simple metric that counts the number of routers a packet must traverse
Bandwidth - Influences path selection by preferring the path with the highest bandwidth
Load - Considers the traffic utilization of a certain link
Delay - Considers the time a packet takes to traverse a path
Reliability - Assesses the probability of a link failure, calculated from the interface error count or previous link failures
Cost - A value determined either by the IOS or by the network administrator to indicate preference for a route. Cost can represent a metric, a combination of metrics or a policy.

Note: At this point, it is not important to completely understand these metrics; they will be explained in later chapters.

3.3.2 - Metrics and Routing Protocols
The animation compares hop count versus bandwidth when used as a metric. The animation is based on the following network topology.

Network Topology:
Three routers, R1, R2, and R3, are interconnected by three WAN links. Router R1 has PC1 connected representing LAN 172.16.3.0/24. Router R2 has PC2 connected representing LAN 172.16.1.0/24.

Link Speeds:
R1 to R2 WAN link bandwidth: 56 kilobits per second
R2 to R3 WAN link bandwidth: T1 (1.544 Megabits per second)
R1 to R3 WAN link bandwidth: T1 (1.544 Megabits per second)

As the animation progresses, PC1 sends a packet to PC2, first using RIP and then using OSPF. RIP chooses the most desirable path based on hop count. Using RIP, R1 sends the packet directly to R2 because R2 is only one hop away. OSPF chooses the most desirable path based on bandwidth. Using OSPF, R1 sends the packet first to R3 and then to R2. Even though using OSPF requires two hops, the packet is sent faster because of increased link speeds between R1 and R3 and between R3 and R2.

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The Metric Field in the Routing Table

The metric for each routing protocol is:

RIP: Hop count - Best path is chosen by the route with the lowest hop count.
IGRP and EIGRP: Bandwidth, Delay, Reliability, and Load - Best path is chosen by the route with the smallest composite metric value calculated from these multiple parameters. By default, only bandwidth and delay are used.
IS-IS and OSPF: Cost - Best path is chosen by the route with the lowest cost. . Cisco's implementation of OSPF uses bandwidth. IS-IS is discussed in CCNP.

Routing protocols determine best path based on the route with the lowest metric.

Refer to the example in the figure The routers are using the RIP routing protocol. The metric associated with a certain route can be best viewed using the show ip route command. The metric value is the second value in the brackets for a routing table entry. In the figure, R2 has a route to the 192.168.8.0/24 network that is 2 hops away.

R 192.168.8.0/24 [120/2] via 192.168.4.1, 00:00:26, Serial0/0/1

Note: More detailed information about specific routing protocol metrics and how to calculate them will be available in the later chapters describing the individual routing protocols.

3.3.2 - Metrics and Routing Protocols
The diagram depicts a metric in the routing table. In the show i p route output for router R2, the hop count appears as two for the RIP route to network 192.168.8.0/24 as highlighted in the output. The routing table entry appears as follows:

R 192.168.8.0/24 [120 / two] via network 192.168.4.1, 00:00:26, Serial0/1

The number two after the [120/ is the metric or, in this case, the hop count.
The diagram is based on the following network topology.

Network Topology:
There are four routers, R1, R2, R3, and R4, five switches, S1, S2, S3, S4, and S5, and five PC's, PC1, PC2, PC3, PC4, and PC5.

PC1 is connected to switch S1, which is connected to R1 interface FA0/0. The network address for this link is 192.168.1.0/24.
PC2 is connected to switch S2, which is connected to R2 interface FA0/0. The network address for this link is 192.168.3.0/24.
PC3 is connected to switch S3, which is connected to R3 interface FA0/0. The network address for this link is 192.168.5.0/24.
PC4 is connected to switch S4, which is connected to R4 interface FA0/0. The network address for this link is 192.168.8.0/24.
PC5 is connected to switch S5, which is connected to R1 interface FA0/1 and R3 interface FA0/1. The network address for this link is 192.168.6.0/24.
R1 interface S0/0/0 is connected to router R2 interface S0/0/0 via a WAN link with a network address of 192.168.2.0/24.
R2 interface S0/0/1 is connected to router R3 interface S0/0/0 via a WAN link with a network address of 192.168.4.0/24.
R3 interface S0/0/1 is connected to router R4 interface S0/0/0 via a WAN link with a network address of 192.168.7.0/24.

3.3.3 Load Balancing

Page 1:
We have discussed that individual routing protocols use metrics to determine the best route to reach remote networks. But what happens when two or more routes to the same destination have identical metric values? How will the router decide which path to use for packet forwarding? In this case, the router does not choose only one route. Instead, the router "load balances" between these equal cost paths. The packets are forwarded using all equal-cost paths.

To see whether load balancing is in effect, check the routing table. Load balancing is in effect if two or more routes are associated with the same destination.

Note: Load balancing can be done either per packet or per destination. How a router actually load balances packets between the equal-cost paths is governed by the switching process. The switching process will be discussed in greater detail in a later chapter.

Play the animation.

R2 load balances traffic to PC5 over two equal cost paths.

The show ip route command reveals that the destination network 192.168.6.0 is available through 192.168.2.1 (Serial 0/0/0) and 192.168.4.1 (Serial 0/0/1).

R 192.168.6.0/24 [120/1] via 192.168.2.1, 00:00:24, Serial0/0/0
[120/1] via 192.168.4.1, 00:00:26, Serial0/0/1

All the routing protocols discussed in this course are capable of automatically load balancing traffic for up to four equal-cost routes by default. EIGRP is also capable of load balancing across unequal-cost paths. This feature of EIGRP is discussed in the CCNP.

3.3.3 - Load Balancing
BRIEF - NOT FOR PROThe animation depicts load balancing across equal cost paths. The route from PC2 to PC5 goes through router R2. There are two equal cost paths to the destination network, one through router R1 and another through R3.

The output from the show i p route command for router R2 shows two routes with the same hop count (one) to network 192.168.6.0/24.

R 192.168.6.0/24 [120 / one] via 192.168.2.1, 00:00:24, Serial0/0/0 [120 / one] via 192.168.4.1, 00:00:26, Serial0/0/1

3.4 Administrative Distances

3.4.1 Purpose of Administrative Distance

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Multiple Routing Sources

We know that routers learn about adjacent networks that are directly connected and about remote networks by using static routes and dynamic routing protocols. In fact, a router might learn of a route to the same network from more than one source. For example, a static route might have been configured for the same network/subnet mask that was learned dynamically by a dynamic routing protocol, such as RIP. The router must choose which route to install.

Note: You might be wondering about equal cost paths. Multiple routes to the same network can only be installed when they come from the same routing source. For example, for equal cost routes to be installed they both must be static routes or they both must be RIP routes.

Although less common, more than one dynamic routing protocol can be deployed in the same network. In some situations it may be necessary to route the same network address using multiple routing protocols such as RIP and OSPF. Because different routing protocols use different metrics, RIP uses hop count and OSPF uses bandwidth, it is not possible to compare metrics to determine the best path.

So, how does a router determine which route to install in the routing table when it has learned about the same network from more than one routing source?

The Purpose of Administrative Distance

Administrative distance (AD) defines the preference of a routing source. Each routing source - including specific routing protocols, static routes, and even directly connected networks - is prioritized in order of most- to least-preferable using an administrative distance value. Cisco routers use the AD feature to select the best path when it learns about the same destination network from two or more different routing sources.

Administrative distance is an integer value from 0 to 255. The lower the value the more preferred the route source. An administrative distance of 0 is the most preferred. Only a directly connected network has an administrative distance of 0, which cannot be changed.

It is possible to modify the administrative distance for static routes and dynamic routing protocols. This is discussed in CCNP.

An administrative distance of 255 means the router will not believe the source of that route and it will not be installed in the routing table.

Note: The term trustworthiness is commonly used when defining administrative distance. The lower the administrative distance value the more trustworthy the route.

3.4.1 - Purpose of Administrative Distance
The diagram compares administrative distance (A D) between RIP and EIGRP. R2 acts as a border router between the EIGRP network (R1 and R2) and the RIP network (R3 and R4). Router R1 is in the EIGRP network, and router R3 is in the RIP network. R1 and R3 do not speak the same routing protocol. The EIGRP network routes have an A D of 90, and the RIP network routes have an A D of 120. The diagram is based on the same network topology as 3.3.2 diagram 2, except with the EIGRP and RIP zones identified.

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Click show ip route in the figure.

The AD value is the first value in the brackets for a routing table entry. Notice that R2 has a route to the 192.168.6.0/24 network with an AD value of 90.

D 192.168.6.0/24 [90/2172416] via 192.168.2.1, 00:00:24, Serial0/0/0

R2 is running both RIP and EIGRP routing protocols. (Remember: it is not common for routers to run multiple dynamic routing protocols, but is used here to demonstrate how administrative distance works.) R2 has learned of the 192.168.6.0/24 route from R1 through EIGRP updates and from R3 through RIP updates. RIP has an administrative distance of 120, but EIGRP has a lower administrative distance of 90. So, R2 adds the route learned using EIGRP to the routing table and forwards all packets for the 192.168.6.0/24 network to router R1.

Click show ip rip database in the figure.

What happens if the link to R1 becomes unavailable? Then R2 would not have a route to 192.168.6.0. Actually, R2 still has the RIP route information for 192.168.6.0 stored in the RIP database. This can be verified with the show ip rip database command. This command shows all RIP routes learned by R2, whether or not the RIP route is installed in the routing table.

3.4.1 - Purpose of Administrative Distance
The diagram compares administrative distances in routing table entries.

Network Topology:
Same as 3.4.1 diagram 1. There are two routes to the 192.168.6.0/24 network. One route is through R1 using EIGRP, and one route is through R3 using RIP.

The output from the show i p route command for router R2 shows a highlighted EIGRP route (preceded by the letter D) to network 192.168.6.0/24 with an A D of 90. There is a potential RIP route to network 192.168.6.0/24, but the EIGRP route has an A D of 90, and the RIP route has an A D of 120. As a result, the EIGRP route, the route with the lower A D, is installed in the routing table. The RIP entry to network 192.168.6.0/24 is highlighted in the output of the show i p rip database command on R2.

R2#show i p route
D 192.168.6.0/24 [90/2172416] via 192.168.2.1, 00:00:24, Serial0/0/0

R2#show i p rip database
192.168.6.0/24
[1] via 192.168.4.1, Serial0/0/1

3.4.2 Dynamic Routing Protocols

Page 1:
Click show ip route in the figure.

You already know that you can verify these AD values with the show ip route command.

Click show ip protocols in the figure.

The AD value can also be verified with the show ip protocols command. This command displays all pertinent information about routing protocols operating on the router. We will look at the show ip protocols command in detail many times during the rest of the course. However, for now notice the highlighted output: R2 has two routing protocols listed and the AD value is called Distance.

Click AD Table in the figure.

Notice the different administrative distance values for various routing protocols.

3.4.2 - Dynamic Routing Protocols
The diagram depicts the default administrative distances for EIGRP and RIP routing table entries

Network Topology:
Same as 3.4.1 diagram 1.

The output from the show i p route command for router R2 shows highlighted EIGRP routes with an A D of 90 and RIP routes with an A D of 120.

The output from the show i p protocols command for router R2 also shows highlighted EIGRP routes with an A D of 90 and RIP routes with an A D of 120.

R2#show i p protocols (highlighted lines)
Routing Protocols is "eigrp 100"
Gateway: 192.168.2.1 Distance: 90

Routing Protocol is rip
Gateway: 192.168.4.1 Distance: 120

A table of A D's from various route sources is provided.
Route Source: Connected
Administrative Distance: 0

Route Source: Static
Administrative Distance: 1

Route Source: EIGRP summary route
Administrative Distance: 5

Route Source: External BGP
Administrative Distance: 20

Route Source: Internal BGP
Administrative Distance: 90

Route Source: IGRP
Administrative Distance: 100

Route Source: OSPF
Administrative Distance: 110

Route Source: I S-I S
Administrative Distance: 115

Route Source: RIP
Administrative Distance: 120

Route Source: External EIGRP
Administrative Distance: 170

Route Source: Internal BGP
Administrative Distance: 200

3.4.3 Static Routes

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As you know from Chapter 2, static routes are entered by an administrator who wants to manually configure the best path to the destination. For that reason, static routes have a default AD value of 1. This means that after directly connected networks, which have a default AD value of 0, static routes are the most preferred route source.

There are situations when an administrator will configure a static route to the same destination that is learned using a dynamic routing protocol, but using a different path. The static route will be configured with an AD greater than that of the routing protocol. If there is a link failure in the path used by the dynamic routing protocol, the route entered by the routing protocol is removed from the routing table. The static route will then become the only source and will automatically be added to the routing table. This is known as a floating static route and is discussed in CCNP.

A static route using either a next-hop IP address or an exit interface has a default AD value of 1. However, the AD value is not listed in show ip route when you configure a static route with the exit interface specified. When a static route is configured with an exit interface, the output shows the network as directly connected via that interface.

Click show ip route in the figure.

The static route to 172.16.3.0 is listed as directly connected. However, there is no information on what the AD value is. It is a common misconception to assume that the AD value of this route must be 0 because it states "directly connected." However, that is a false assumption. The default AD of any static route, including those configured with an exit interface is 1. Remember, only a directly connected network can have an AD of 0. This can be verified by extending the show ip route command with the [route] option. Specifying the [route] reveals detailed information about the route, including its distance, or AD value.

Click show ip route 172.16.3.0 in the figure.

The command show ip route 172.16.3.0 reveals that, in fact, the administrative distance is 1.

3.4.3 - Static Routes
The diagram depicts administrative distance and static routes. The diagram is based on the following network topology.

Network Topology:
There are three routers, R1, R2, and R3, three switches, S1, S2, and S3, and three PC's, PC1, PC2, and PC3.

PC1 is connected to LAN switch S1, which is connected to R1 interface FA0/0. R1 interface S0/0/0 (DCE) is connected to router R2 interface S0/0/0 via a WAN link.

PC2 is connected to LAN switch S2, which is connected to R2 interface FA0/0. R2 interface S0/0/0 is connected to router R1, and R2 interface S0/0/1 (DCE) is connected to router R3 via WAN links.

PC3 is connected to LAN switch S3, which is connected to R3 interface FA0/0. R3 interface S0/0/1 is connected to router R2 via a WAN link.

Topology Network Addresses:
R1 LAN network address: 172.16.3.0/24
R1 to R2 WAN network address: 172.16.2.0/24
R2 LAN network address: 172.16.1.0/24
R2 to R3 WAN network address: 192.168.1.0/24
R3 LAN network address: 192.168.2.0/24

The output from the show i p route command for router R2 shows the following highlighted entries for static routes:
S 172.16.3.0 is directly connected, Serial0/0/0
S 192.168.2.0/24 [1/0] via 192.168.1.1

The output from the show i p route 172.16.3.0 command for router R2 shows the following entry.
Known via static, distance 1, metric 0 (connected)

3.4.4 Directly Connected Networks

Page 1:
Directly connected networks appear in the routing table as soon as the IP address on the interface is configured and the interface is enabled and operational. The AD value of directly connected networks is 0, meaning that this is the most preferred routing source. There is no better route for a router than having one of its interfaces directly connected to that network. For that reason, the administrative distance of a directly connected network cannot be changed and no other route source can have an administrative distance of 0.

Click show ip route in the figure.

The output of the show ip route command displays the directly connected networks with no information about the AD value. The output is similar to the output for static routes that point to an exit interface. The only difference is the letter C at the beginning of the entry, which indicates that this is a directly connected network.

To see the AD value of a directly connected network, use the [route] option.

Click show ip route 172.16.1.0 in the figure.

The show ip route 172.16.1.0 command reveals that the distance is 0 for that directly connected route.

3.4.4 - Directly Connected Networks
The diagram depicts administrative distance and directly connected networks.

Network Topology:
Same as 3.4.3 diagram 1.

The output from the show i p route command for router R2 shows the following highlighted entries for directly connected routes:
C 172.16.1.0 is directly connected, FastEthernet0/0
C 172.16.2.0 is directly connected, Serial0/0/0
C 192.168.1.0/24 is directly connect, Serial0/0/1

The output from the show i p route 172.16.1.0 command for router R2 shows the following entry:
Known via connected, distance 0, metric 0 (connected via interface)

Page 2:
In this activity, you will use version of the show ip route command to see details of routing table entries.

Click the Packet Tracer icon for more details.

3.4.4 - Directly Connected Networks
Link to Packet Tracer Exploration: Viewing Routing Table Information - show i p route

In this activity, you use a version of the show i p route command to see details of routing table entries.

3.5 Routing Protocols and Subnetting Activities

3.5.1 Identifying Elements of the Routing Table

Page 1:
The purpose of this exercise is to practice how to correctly identify the route source, administrative distance, and metric for a given route based on output from the show ip route command.

The output is not common for most routing tables. Running more than one routing protocol on the same router is rare. Running three, as shown here, is more of an academic exercise and has value in that it will help you learn to interpret the routing table output.

Drag and drop the appropriate responses to the corresponding space in the table.

Use the information from the Show IP Route as reference.
Not all answers are used.
Some answers are used more than once.

3.5.1 - Identify Elements of the Routing Table
The diagram depicts an activity in which you must correctly identify the route source, administrative distance, and metric for a given route based on output from the show i p route command.

Note: Contact your instructor for assistance with this activity.

Select the proper entry to fill in the blanks in the following table. Use the information from the show i p route as a reference. Not all answers are used.
Some answers are used more than once.

R2#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
I - 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

10.0.0.0/16 is subnetted, 1 subnets
S 10.4.0.0 is directly connected, Serial0/0
172.16.0.0/24 is subnetted, 3 subnets
C 172.16.1.0 is directly connected, FastEthernet0/0
C 172.16.2.0 is directly connected, Serial0/0
D 172.16.3.0 [90/2172416] via 172.16.2.1, 00:00:18, Serial0/0
C 192.168.1.0/24 is directly connected, Serial0/1
O 192.168.100.0/24 [110/65] via 172.16.2.1, 00:00:03, Serial0/0
O 192.168.110.0/24 [110/65] via 172.16.2.1, 00:00:03, Serial0/0
R 192.168.120.0/24 [110/65] via 172.16.2.1, 00:00:18, Serial0/0

Routes for which to identify information:
Route: 10.4.0.0/16
Route source: BLANK
A D: BLANK
Metric: BLANK

Route: 172.16.2.0/24
Route source: BLANK
A D: BLANK
Metric: BLANK

Route: 172.16.1.0/24
Route source: BLANK
A D: BLANK
Metric: BLANK

Route: 172.16.3.0/24
Route source: BLANK
A D: BLANK
Metric: BLANK

Route: 192.168.1.0/24
Route source: BLANK
A D: BLANK
Metric: BLANK

Route: 192.168.100.0/24
Route source: BLANK
A D: BLANK
Metric: BLANK

Route: 192.168.110.0/24
Route source: BLANK
A D: BLANK
Metric: BLANK

Route: 192.168.120.0/24
Route source: BLANK
A D: BLANK
Metric: BLANK

Entries for use in filling in the blanks:
EIGRP.
RIP.
OSPF.
ODR.
BGP.
Static.
Connected.
One.
65.
Zero.
110.
90.
120.
2172416.

3.5.2 Subnetting Scenario 1

Page 1:
In this activity, you have been given the network address 192.168.9.0/24 to subnet and provide the IP addressing for the network shown in the Topology Diagram.

Click the lab icon for more details.

3.5.2 - Subnetting Scenario 1
Link to Hands-on Lab: Subnetting Scenario 1

In this activity, you have been given the network address 192.168.9.0/24 to subnet and provide the IP addressing for the network shown in the Topology Diagram.

Page 2:
Use this Packet Tracer Activity to implement your addressing scheme.

A summary of the instructions are provided within the activity. Use the Lab PDF for more details.

Click the Packet Tracer icon for more details.

3.5.2 - Subnetting Scenario 1
Link to Packet Tracer Exploration: Subnetting Scenario 1 with Static Routing

Use this Packet Tracer Activity to implement your addressing scheme.

3.5.3 Subnetting Scenario 2

Page 1:
In this activity, you have been given the network address 172.16.0.0/16 to subnet and provide the IP addressing for the network shown in the Topology Diagram.

Click the lab icon for more details.

3.5.3 - Subnetting Scenario 2
Link to Hands-on Lab: Subnetting Scenario 2

In this activity, you have been given the network address 172.16.0.0/16 to subnet and provide the IP addressing for the network shown in the Topology Diagram.

3.5.3 - Subnetting Scenario 2
Link to Packet Tracer Exploration: Subnetting Scenario 2 with Static Routing

Use this Packet Tracer Activity to implement your addressing scheme.

3.5.4 Subnetting Scenario 3

Page 1:
In this activity, you have been given the network address 192.168.1.0/24 to subnet and provide the IP addressing for the network shown in the Topology Diagram.

Click the lab icon for more details.

3.5.4 - Subnetting Scenario 3
Link to Hands-on Lab: Subnetting Scenario 3

In this activity, you have been given the network address 192.168.1.0/24 to subnet and provide the IP addressing for the network shown in the Topology Diagram.

3.5.4 - Subnetting Scenario 3
Link to Packet Tracer Exploration: Subnetting Scenario 3

Use this Packet Tracer Activity to implement your addressing scheme.

3.6 Summary

3.6.1 Summary and Review

Page 1:
Summary

Dynamic routing protocols are used by routers to automatically learn about remote networks from other routers. In this chapter you were introduced to several different dynamic routing protocols.

You learned that routing protocols can be classified as either classful or classless, either distance vector, link-state, or path vector, and whether a routing protocol is an interior gateway protocol or an exterior gateway protocol. The differences in these classifications will become better understood as you learn more about these routing concepts and protocols in later chapters.

Routing protocols not only discover remote networks, but also have a procedure for maintaining accurate network information. When there is a change in the topology it is the function of the routing protocol to inform other routers about this change.

When there is a change in the network topology, some routing protocols can propagate that information throughout the routing domain faster than other routing protocols. The process of bringing all routing tables to a state of consistency is called convergence. Convergence is when all of the routers in the same routing domain or area have complete and accurate information about the network.

Metrics are used by routing protocols to determine the best path or shortest path to reach a destination network. Different routing protocols may use different metrics. Typically, a lower metric means a better path. Five hops to reach a network is better than 10 hops.

Routers sometimes learn about multiple routes to the same network from both static routes and dynamic routing protocols. When a router learns about a destination network from more than one routing source, Cisco routers use the administrative distance value to determine which source to use. Each dynamic routing protocol has a unique administrative value, along with static routes and directly connected networks. The lower the administrative value, the more preferred the route source. A directly connected network is always the preferred source, followed by static routes and then various dynamic routing protocols.

All of the classifications and concepts in this chapter will be discussed more thoroughly in the rest of the chapters of this course. At the end of this course you may wish to review this chapter to get a review and overview of this information.

3.6.1 - Summary and Review
In this chapter, you learned to:
- Describe the role of dynamic routing protocols and place these protocols in the context of modem network design.
- Identify several ways to classify routing protocols.
- Describe how metrics are used by routing protocols, and identify the metric types used by dynamic routing protocols.
- Determine the administrative distance of a route and describe its importance in the routing process. Identify the different elements in the routing table.
- Given realistic constraints, devise and apply subnetting schemes.

Page 2:

3.6.1 - Summary and Review
This is a review and is not a quiz. Questions and answers are provided.
Question 1. Explain why static routing might be preferred over dynamic routing.
Answer: Static routing is more secure, uses less router computational power, and is easier to understand. It is more secure because routers do not advertise routing information to other routers. It uses less router resources than dynamic routing, which requires the implementation of algorithms and the processing of update packets. It is often easier to understand than some of the more complex routing protocols.

Question 2. What are four ways of classifying dynamic routing protocols?
Answer: Dynamic routing protocols can be classified as either interior or exterior, distance vector or link-state, classful or classless, and by speed of convergence.

Question 3. What are the most common metrics used in IP dynamic routing protocols?
Answer: Hop count, bandwidth, delay, and cost.

Question 4. What is administrative distance and why is it important?
Answer: Administrative distance is a measure of the trustworthiness of a route source. It is used when a router has learned routes to the same destination from two different route sources. It is important because not all route sources are equal. For example, you would not want a router sending traffic to another router if the destination is a directly connected network! Administrative distance ensures that this does not happen because directly connected routes are trusted over all other route sources.

Question 5. It can be said that every router must have at least one static route. Explain why this statement might be true.
Answer: Every router that forwards user traffic to the Internet has at least one static route. That static route is a default route. Every household that has a router to connect to the Internet uses a static default route to send all traffic to the ISP.

Question 6. Students new to routing sometimes assume that bandwidth is a better metric than hop count. Why might this be a false assumption?
Answer: Hop count might actually use the better path than the path chosen by a bandwidth metric in the event that one of the paths is saturated with traffic.

Page 3:
The Packet Tracer Skills Integration Challenge Activity for this chapter is very similar to the activity you completed at the end of Chapter 2. The scenario is slightly different, allowing you to better practice your skills. 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. Then you must implement an effective static routing configuration.

Packet Tracer Skills Integration Instructions (PDF)

Click the Packet Tracer icon for more details.

3.6.1 - Summary and Review
Link to Packet Tracer Exploration: Chapter 3 - Packet Tracer Skills Integration Challenge

The Packet Tracer Skills Integration Challenge Activity for this chapter is very similar to the activity you completed at the end of Chapter 2. The scenario is slightly different, allowing you to better practice your skills. 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. Then you must implement an effective static routing configuration.

Page 4:
To Learn More

Border Gateway Protocol (BGP) is an inter-autonomous routing protocol - the routing protocol of the Internet. Although BGP is only briefly discussed in this course (it is discussed more fully in CCNP), you might find it interesting to view routing tables of some of the Internet core routers.

Route servers are used to view BGP routes on the Internet. Various web sites provide access to these route server, for example www.traceroute.org. When choosing a route server in a specific autonomous system, you will start a telnet session on that route server. This server is mirroring an Internet core router which is most often a Cisco router.

You can then use the show ip route command to view the actual routing table of an Internet router. Use the show ip route command followed by the public or global network address of your school, for example show ip route 207.62.187.0.

You will not be able to understand much of the information in this output, but these commands should give you a sense of the size of a routing table on a core Internet router.

3.6.1 - Summary and Review
The diagram depicts a collage of people using computers and networks.

3.7 Chapter Quiz

3.7.1 Chapter Quiz

Page 1:

3.7.1 - Chapter Quiz
1. What are two advantages of static routing over dynamic routing? (Choose two.)
A. Configuration is less error prone.
B. More secure because routers do not advertise routes.
C. Growing the network usually does not present a problem.
D. No computing overhead.
E. Administrator has less work maintaining the configuration.

2. Match the routing protocol to its description.
Protocols:
RIP
IGRP
OSPF
EIGRP
BGP

Descriptions:
distance vector exterior routing protocol
Cisco's advanced interior routing protocol
link-state interior routing protocol
distance vector interior routing protocol
Cisco's distance vector interior routing protocol

3. Which statement best describes convergence on a network?
A. The amount of time required for routers to share administrative configuration changes, such as password changes, from one end of a network to the other end.
B. The time required for the routers in the network to update their routing tables after a topology change has occurred.
C. The time required for the routers in one autonomous system to learn routes to destinations in another autonomous system.
D. The time required for routers running disparate routing protocols to update their routing tables.

4. Which two parameters are used to calculate metrics? (Choose two.)
A. hop count
B. uptime
C. bandwidth
D. convergence time
E. administrative distance

5. Which routing protocol has the most trustworthy administrative distance by default?
A. EIGRP internal routes
B. I S-I S
C. OSPF
D. RIP v1
E. RIP v2

6. How many equal cost paths can a dynamic routing protocol use for load balancing by default?
A. 2
B. 3
C. 4
D. 6

7. Which command shows the administrative distance of routes?
A. R1#show interfaces
B. R1#show i p route
C. R1#show i p interfaces
D. R1#debug i p routing

8. When do directly connected networks appear in the routing table?
A. When they are included in a static route.
B. When they are used as an exit interface.
C. As soon as they are addressed as operational at Layer 2.
D. As soon as they are addressed and operational at Layer 3.
E. Always when a no shutdown command is issued.

9. Router1 is using the RIP v2 routing protocol and has multiple unequal paths to reach a destination network. How does Router1 determine which path is the best path to the destination network?
A. lowest metric
B. highest metric
C. lowest administrative distance
D. highest administrative distance
E. load balancing between up to four paths

10. Match the administrative distance to the corresponding routing protocol.
Administrative Distances:
5
20
90
100
110
115
120
170
200

Protocols:
eBGP
EIGRP (Internal)
EIGRP (External)
I S-I S
OSPF
RIP

11. Indicate whether the characteristic is related to classful or classless routing. Not all characteristics apply.

A. Does not support discontiguous networks (classful, classless, or neither)
B. Supported by EIGRP, OSPF, and BGP (classful, classless, or neither)
C. Sends subnet mask in its routing updates (classful, classless, or neither)
D. Supports discontiguous networks (classful, classless, or neither)
E. Supported by RIP version 1 and IGRP (classful, classless, or neither)
F. Does not send the subnet mask in its routing updates (classful, classless, or neither)
G. Cisco proprietary (classful, classless, or neither)
H. Open standard (classful, classless, or neither)

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