4 Distance Vector Routing Protocols

4.0 Chapter Introduction

4.0.1 Chapter Introduction

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Introduction

The dynamic routing chapters of this course focus on Interior Gateway Protocols (IGPs). As discussed in Chapter 3, IGPs are classified as either distance vector or link-state routing protocols.

This chapter describes the characteristics, operations, and functionality of distance vector routing protocols. There are advantages and disadvantages to using any type of routing protocol. Therefore, the conditions influencing the operation of distance vector protocols and the pitfalls of distance vector protocol operation - along with remedies to overcome such pitfalls - are described. Understanding the operation of distance vector routing is critical to enabling, verifying, and troubleshooting these protocols.


4.0.1 - Chapter Introduction
The diagram depicts the dynamic interior gateway routing protocols that are covered in this course. These include RIP, RIP v2, EIGRP, and OSPF v2. In this chapter you will learn to:
- Identify the characteristics of distance vector routing protocols.
- Describe the network discovery process of distance vector routing protocols using Routing Information Protocol (RIP).
- Describe the processes for maintaining accurate routing tables that are used by distance vector routing protocols.
- Identify the conditions leading to a routing loop and explain the implications for router performance.
- Identify the types of distance vector routing protocols in use today.


4.1 Introduction to Distance Vector Routing Protocols

4.1.1 Distance Vector Routing Protocols

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Dynamic routing protocols help the network administrator overcome the time-consuming and exacting process of configuring and maintaining static routes. For example, can you imagine maintaining the static routing configurations of the 28 routers shown in the figure? What happens when a link goes down? How do you ensure that redundant paths are available? Dynamic routing is the most common choice for large networks like the one shown.

Distance vector routing protocols include RIP, IGRP, and EIGRP.

RIP

Routing Information Protocol (RIP) was originally specified in RFC 1058. It has the following key characteristics:
  • Hop count is used as the metric for path selection.
  • If the hop count for a network is greater than 15, RIP cannot supply a route to that network.
  • Routing updates are broadcast or multicast every 30 seconds, by default.
IGRP

Interior Gateway Routing Protocol (IGRP) is a proprietary protocol developed by Cisco. IGRP has the following key design characteristics:
  • Bandwidth, delay, load and reliability are used to create a composite metric.
  • Routing updates are broadcast every 90 seconds, by default.
  • IGRP is the predecessor of EIGRP and is now obsolete.
EIGRP

Enhanced IGRP (EIGRP) is a Cisco proprietary distance vector routing protocol. EIGRP has these key characteristics:
  • It can perform unequal cost load balancing.
  • It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path.
  • There are no periodic updates as with RIP and IGRP. Routing updates are sent only when there is a change in the topology.


4.1.1 - Distance Vector Routing Protocols
The diagram depicts a complex network with many routers connected in four different regions. Imagine maintaining static routing configurations for this network!


4.1.2 Distance Vector Technology

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The Meaning of Distance Vector

As the name implies, 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.

A router using a distance vector routing protocol does not have the knowledge of the entire path to a destination network. Instead the router knows only:
  • The direction or interface in which packets should be forwarded and
  • The distance or how far it is to the destination network
For example, in the figure, R1 knows that the distance to reach network 172.16.3.0/24 is 1 hop and that the direction is out the interface S0/0/0 toward R2.


4.1.2 - Distance Vector Technology
The diagram depicts the meaning of distance vector. Distance is how far away a destination network is, and vector is in what direction packets should be forwarded. The image is based on the following network topology.

Network Topology:
Router R1 interface S0/0/0 is connected to router R2 via a WAN link. LAN 172.16.3.0/24 is connected to R2 interface FA0/0.

Router R1 is one hop away (distance) from network 172.16.3.0/24. It can be reached through router R2 (vector).


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Operation of Distance Vector Routing Protocols

Some distance vector routing protocols call for the router to periodically broadcast the entire routing table to each of its neighbors. This method is inefficient because the updates not only consume bandwidth but also consume router CPU resources to process the updates.

Distance vector routing protocols share certain characteristics.

Periodic Updates are sent at regular intervals (30 seconds for RIP and 90 seconds for IGRP). Even if the topology has not changed in several days, periodic updates continue to be sent to all neighbors.

Neighbors are routers that share a link and are configured to use the same routing protocol. The router is only aware of the network addresses of its own interfaces and the remote network addresses it can reach through its neighbors. It has no broader knowledge of the network topology. Routers using distance vector routing are not aware of the network topology.

Broadcast Updates are sent to 255.255.255.255. Neighboring routers that are configured with the same routing protocol will process the updates. All other devices will also process the update up to Layer 3 before discarding it. Some distance vector routing protocols use multicast addresses instead of broadcast addresses.

Entire Routing Table Updates are sent, with some exceptions to be discussed later, periodically to all neighbors. Neighbors receiving these updates must process the entire update to find pertinent information and discard the rest. Some distance vector routing protocols like EIGRP do not send periodic routing table updates.


4.1.2 - Distance Vector Technology
The animation depicts four routers interconnected by WAN links and demonstrates how distance vector periodic updates take place between routers. Each router sends an update to its neighboring routers based on a timer.


4.1.3 Routing Protocol Algorithms

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The Purpose of the Algorithm

At the core of the distance vector protocol is the algorithm. The algorithm is used to calculate the best paths and then send that information to the neighbors.

An algorithm is a procedure for accomplishing a certain task, starting at a given initial state and terminating in a defined end state. Different routing protocols use different algorithms to install routes in the routing table, send updates to neighbors, and make path determination decisions.

The algorithm used for the routing protocols defines the following processes:
  • Mechanism for sending and receiving routing information.
  • Mechanism for calculating the best paths and installing routes in the routing table.
  • Mechanism for detecting and reacting to topology changes.
In the animation, R1 and R2 are configured with a routing protocol. The algorithm sends and receives updates. Both R1 and R2 then glean new information from the update. In this case, each router learns about a new network. The algorithm on each router makes its calculations independently and updates the routing table with the new information. When the LAN on R2 goes down, the algorithm constructs a "triggered" update and sends it to R1. R1 then removes the network from the routing table. Triggered updates will be discusses later in this chapter.


4.1.3 - Routing Protocol Algorithms
The diagram depicts the purposes of routing algorithms. They include:
- Sending and receiving updates
- Calculate best path and install routes
- Detect and react to topology changes
The animation is based on the following network topology.

Network Topology:
LAN 172.16.1.0/24 is connected to R1 interface FA0/0. Router R1 interface S0/0/0 is connected to router R2 interface S0/0/0 via WAN link network 172.16.2.0/24. LAN 172.16.3.0/24 is connected to R2 interface FA0/0.

As the animation progresses, router R1's routing protocol builds an update containing its LAN 172.16.1.0/24 and sends it to R2. R2 installs the route in its routing table. Router R2's routing protocol builds an update that contains its LAN 172.16.3.0/24 and sends it to R1. R1 installs the route in its routing table. The R2 link to LAN 172.16.3.0/24 goes down. The R2 routing protocol removes the route from its routing table, builds an update that omits its LAN 172.16.3.0/24, and sends it to R1. R1 removes the route from its routing table.


4.1.4 Routing Protocol Characteristics

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Routing Protocols Characteristics

Routing protocols can be compared based on the following characteristics:
  • Time to Convergence - Time to convergence defines how quickly the routers in the network topology share routing information and reach a state of consistent knowledge. The faster the convergence, the more preferable the protocol. Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.
  • Scalability - Scalability defines how large a network can become based on the routing protocol that is deployed. The larger the network is, the more scalable the routing protocol needs to be.
  • Classless (Use of VLSM) or Classful - Classless routing protocols include the subnet mask in the updates. This feature supports the use of Variable Length Subnet Masking (VLSM) and better route summarization. Classful routing protocols do not include the subnet mask and cannot support VLSM.
  • Resource Usage - Resource usage includes the requirements of a routing protocol such as memory space, CPU utilization, and link bandwidth utilization. Higher resource requirements necessitate more powerful hardware to support the routing protocol operation in addition to the packet forwarding processes.
  • Implementation and Maintenance - Implementation and maintenance describes the level of knowledge that is required for a network administrator to implement and maintain the network based on the routing protocol deployed.
The advantages and disadvantages of distance vector routing protocols are shown in the table.


4.1.4 - Routing Protocol Characteristics
The diagram depicts advantages and disadvantages of distance vector routing protocols.

Advantages:
- Simple implementation and maintenance. The level of knowledge required to deploy and later maintain a network with distance vector protocol is not high.

- Low resource requirements. Distance vector protocols typically do not need large amounts of memory to store the information. Nor do they require a powerful CPU. Depending on the network size and the IP addressing implemented, they also typically do not require a high level of link bandwidth to send routing updates. However, this can become an issue if you deploy a distance vector protocol in a large network.

Disadvantages:
- Slow convergence. The use of periodic updates can cause slower convergence. Even if some advanced techniques are used, like triggered updates, which are discussed later, the overall convergence is still slower compared to link-state routing protocols.

- Limited scalability. Slow convergence can limit the size of the network because larger networks require more time to propagate routing information.

- Routing loops. Routing loops can occur when inconsistent routing tables are not updated because of slow convergence in a changing network.


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Routing Protocol Learning Check

In the figure, all the routing protocols discussed in the course are compared based on these characteristics. Although IGRP is no longer supported by the IOS, it is shown here to compare it with the Enhanced version. Also, although the IS-IS routing protocol is covered in the CCNP courses, it is shown here because it is a commonly used interior gateway protocol.

Study the figure, and then click the Reset button to empty the table. Drag and drop the appropriate characteristics to each routing protocol. Based on the information previously discussed, you should be able to identify the advantages and disadvantages of distance vector routing protocols.


4.1.4 - Routing Protocol Characteristics
The diagram depicts a routing protocol learning check activity in which you replace each blank with a given term as it relates to the characteristics of a routing protocol.

Terms for filling in the blanks (terms can be used more than once):
Slow
Fast
No
Yes
Low
High
Small
Large
Simple
Complex
Medium

Distance Vector Protocols and Characteristics:
RIP v1:
Speed of convergence: BLANK
Scalability - Size of network: BLANK
Use of VLSM: BLANK
Resource usage: BLANK
Implementation and maintenance: BLANK

RIP v2:
Speed of convergence: BLANK
Scalability - Size of network: BLANK
Use of VLSM: BLANK
Resource usage: BLANK
Implementation and maintenance: BLANK

IGRP:
Speed of convergence: BLANK
Scalability - Size of network: BLANK
Use of VLSM: BLANK
Resource usage: BLANK
Implementation and maintenance: BLANK

EIGRP:
Speed of convergence: BLANK
Scalability - Size of network: BLANK
Use of VLSM: BLANK
Resource usage: BLANK
Implementation and maintenance: BLANK

Link State Protocols Characteristics:
OSPF:
Speed of convergence: BLANK
Scalability - Size of network: BLANK
Use of VLSM: BLANK
Resource usage: BLANK
Implementation and maintenance: BLANK

I S-I S:
Speed of convergence: BLANK
Scalability - Size of network: BLANK
Use of VLSM: BLANK
Resource usage: BLANK
Implementation and maintenance: BLANK


4.2 Network Discovery

4.2.1 Cold Start

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When a router cold starts or powers up, it knows nothing about the network topology. It does not even know that there are devices on the other end of its links. The only information that a router has is from its own saved configuration file stored in NVRAM. Once a router boots successfully, it applies the saved configuration. As described in Chapter 1 and Chapter 2, if the IP addressing is configured correctly, then the router will initially discover its own directly connected networks.

Initial Network Discovery

In the example in the figure, after a cold start and before the exchange of routing information, the routers initially discover their own directly connected networks and subnet masks. This information is added to their routing tables:

R1
  • 10.1.0.0 available through interface FastEthernet 0/0
  • 10.2.0.0 available through interface Serial 0/0/0
R2
  • 10.2.0.0 available through interface Serial 0/0/0
  • 10.3.0.0 available through interface Serial 0/0/1
R3
  • 10.3.0.0 available through interface Serial 0/0/1
  • 10.4.0.0 available through interface FastEthernet 0/0
Play the animation to watch this initial discovery of connected networks for R1. With this initial information, the routers start to exchange routing information.


4.2.1 - Cold Start
The animation depicts router network discovery from a cold start. The animation is based on the following network topology.

Network Topology:
There are three routers, R1, R2, and R3, connected via WAN links.
Router R1 FA0/0 is connected to LAN 10.1.0.0.
Router R1 S0/0/0 is connected to router R2 S0/0/0 via WAN 10.2.0.0.
Router R2 S0/0/1 is connected to router R3 S0/0/1 via WAN 10.3.0.0.
Router R3 FA0/0 is connected to LAN 10.4.0.0.

As the animation progresses, and the routers start up, they discover directly connected networks and build their routing tables as follows:

Router R1 routing table:
Network: 10.1.0.0
Interface: FA0/0
Hop: 0

Network: 10.2.0.0
Interface: S0/0/0
Hop: 0

Router R2 routing table:
Network: 10.2.0.0
Interface: S0/0/0
Hop: 0

Network: 10.3.0.0
Interface: S0/0/1
Hop: 0

Router R3 routing table:
Network: 10.3.0.0
Interface: S0/0/1
Hop: 0

Network: 10.4.0.0
Interface: FA0/0
Hop: 0


4.2.2 Initial Exchange of Routing Information

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If a routing protocol is configured, the routers begin exchanging routing updates. Initially, these updates only include information about their directly connected networks. Upon receiving an update, the router checks it for new information. Any routes that are not currently in its routing table are added.

Initial Exchange

Play the animation to see R1, R2, and R3 start the initial exchange. All three routers send their routing tables to their neighbors, which at this point only contains the directly connected networks. Each router processes updates in the following manner:

R1
  • Sends an update about network 10.1.0.0 out the Serial0/0/0 interface
  • Sends an update about network 10.2.0.0 out the FastEthernet0/0 interface
  • Receives update from R2 about network 10.3.0.0 with a metric of 1
  • Stores network 10.3.0.0 in the routing table with a metric of 1
R2
  • Sends an update about network 10.3.0.0 out the Serial 0/0/0 interface
  • Sends an update about network 10.2.0.0 out the Serial 0/0/1 interface
  • Receives an update from R1 about network 10.1.0.0 with a metric of 1
  • Stores network 10.1.0.0 in the routing table with a metric of 1
  • Receives an update from R3 about network 10.4.0.0 with a metric of 1
  • Stores network 10.4.0.0 in the routing table with a metric of 1
R3
  • Sends an update about network 10.4.0.0 out the Serial 0/0/1 interface
  • Sends an update about network 10.3.0.0 out the FastEthernet0/0
  • Receives an update from R2 about network 10.2.0.0 with a metric of 1
  • Stores network 10.2.0.0 in the routing table with a metric of 1
After this first round of update exchanges, each router knows about the connected networks of their directly connected neighbors. However, did you notice that R1 does not yet know about 10.4.0.0 and that R3 does not yet know about 10.1.0.0? Full knowledge and a converged network will not take place until there is another exchange of routing information.


4.2.2 - Initial Exchange of Routing Information
The animation depicts router network discovery from the initial routing protocol update exchange. The animation is based on the network topology described in 4.2.1 diagram 1.

As the animation progresses, the routing protocol in each router builds updates from the directly connected routes and sends the first round of updates to the other routers. The other routers add the newly learned routes to their routing tables, resulting in the following routing table entries for each router:

Router R1 routing table:
Network: 10.1.0.0
Interface: FA0/0
Hop: 0

Network: 10.2.0.0
Interface: S0/0/0
Hop: 0

Network: 10.3.0.0
Interface: S0/0/0
Hop: 1

Router R2 routing table:
Network: 10.2.0.0
Interface: S0/0/0
Hop: 0

Network: 10.3.0.0
Interface: S0/0/1
Hop: 0

Network: 10.1.0.0
Interface: S0/0/0
Hop: 1

Network: 10.4.0.0
Interface: S0/0/1
Hop: 1

Router R3 routing table:
Network: 10.3.0.0
Interface: S0/0/1
Hop: 0

Network: 10.4.0.0
Interface: FA0/0
Hop: 0

Network: 10.2.0.0
Interface: S0/0/1
Hop: 1


4.2.3 Exchange of Routing Information

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At this point the routers have knowledge about their own directly connected networks and about the connected networks of their immediate neighbors. Continuing the journey toward convergence, the routers exchange the next round of periodic updates. Each router again checks the updates for new information.

Next Update

Play the animation to see R1, R2, and R3 send the latest routing table to their neighbors. Each router processes updates in the following manner:

R1
  • Sends an update about network 10.1.0.0 out the Serial 0/0/0 interface.
  • Sends an update about networks 10.2.0.0 and 10.3.0.0 out the FastEthernet0/0 interface.
  • Receives an update from R2 about network 10.4.0.0 with a metric of 2.
  • Stores network 10.4.0.0 in the routing table with a metric of 2.
  • Same update from R2 contains information about network 10.3.0.0 with a metric of 1. There is no change; therefore, the routing information remains the same.
R2
  • Sends an update about networks 10.3.0.0 and 10.4.0.0 out of Serial 0/0/0 interface.
  • Sends an update about networks 10.1.0.0 and 10.2.0.0 out of Serial 0/0/1 interface.
  • Receives an update from R1 about network 10.1.0.0. There is no change; therefore, the routing information remains the same.
  • Receives an update from R3 about network 10.4.0.0. There is no change; therefore, the routing information remains the same.
R3
  • Sends an update about network 10.4.0.0 out the Serial 0/0/1 interface.
  • Sends an update about networks 10.2.0.0 and 10.3.0.0 out the FastEthernet0/0 interface.
  • Receives an update from R2 about network 10.1.0.0 with a metric of 2.
  • Stores network 10.1.0.0 in the routing table with a metric of 2.
  • Same update from R2 contains information about network 10.2.0.0 with a metric of 1. There is no change; therefore, the routing information remains the same.
Note: Distance vector routing protocols typically implement a technique known as split horizon. Split horizon prevents information from being sent out the same interface from which it was received. For example, R2 would not send an update out Serial 0/0/0 containing the network 10.1.0.0 because R2 learned about that network through Serial 0/0/0. This mechanism will be explained in more detail later in this chapter.


4.2.3 - Exchange of Routing Information
The animation depicts router network discovery from the next routing protocol update exchange. The animation is based on the network topology described in 4.2.1 diagram 1.

As the animation progresses, the routing protocol in R2 sends the second round of updates to routers R1 and R3. R1 and R3 add the newly learned routes to their routing tables, resulting in the following routing table entries for each router:

Router R1 routing table:
Network: 10.1.0.0
Interface: FA0/0
Hop: 0

Network: 10.2.0.0
Interface: S0/0/0
Hop: 0

Network: 10.3.0.0
Interface: S0/0/0
Hop: 1

New entry:
Network: 10.4.0.0
Interface: S0/0/0
Hop: 2

Router R2 routing table:
Network: 10.2.0.0
Interface: S0/0/0
Hop: 0

Network: 10.3.0.0
Interface: S0/0/1
Hop: 0

Network: 10.1.0.0
Interface: S0/0/0
Hop: 1

Network: 10.4.0.0
Interface: S0/0/1
Hop: 1

Router R3 routing table:
Network: 10.3.0.0
Interface: S0/0/1
Hop: 0

Network: 10.4.0.0
Interface: FA0/0
Hop: 0

Network: 10.2.0.0
Interface: S0/0/1
Hop: 1

New entry:
Network: 10.1.0.0
Interface: S0/0/1
Hop: 2


4.2.4 Convergence

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The amount of time it takes for a network to converge is directly proportional to the size of that network. In the animation, a branch router in Region 4 (B2-R4) is cold starting. The animation shows the propagation of new routing information as updates are sent between neighboring routers. It takes five rounds of periodic update intervals before most of the branch routers in Regions 1, 2, and 3 learn about the new routes advertised by B2-R4. Routing protocols are compared based on how fast they can propagate this information - their speed to convergence.

The speed of achieving convergence consists of:
  • How quickly the routers propagate a change in the topology in a routing update to its neighbors.
  • The speed of calculating best path routes using the new routing information collected.
A network is not completely operable until it has converged, therefore, network administrators prefer routing protocols with shorter convergence times.


4.2.4 - Convergence
The animation depicts convergence time with a large complex network and many routers connected in four different regions. It takes five rounds of periodic update intervals before most of the branch routers in regions 1, 2, and 3 learn about the new routes advertised by a distant router in region 4.


4.3 Routing Table Maintenance

4.3.1 Periodic Updates: RIPv1 and IGRP

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Maintaining the Routing Table

Many distance vector protocols employ periodic updates to exchange routing information with their neighbors and to maintain up-to-date routing information in the routing table. RIP and IGRP are examples of two such protocols.

In the animation, the routers are periodically sending the routing table to neighbors. The term periodic updates refers to the fact that a router sends the complete routing table to its neighbors at a predefined interval. For RIP, these updates are sent every 30 seconds as a broadcast (255.255.255.255) whether or not there has been a topology change. This 30-second interval is a route update timer that also aids in tracking the age of routing information in the routing table.

The age of routing information in a routing table is refreshed each time an update is received. This way information in the routing table can be maintained when there is a topology change. Changes may occur for several reasons, including:
  • Failure of a link
  • Introduction of a new link
  • Failure of a router
  • Change of link parameters


4.3.1 - Periodic Updates: RIP v1 and IGRP
The animation depicts periodic updates where each router passes its entire routing table to its neighbor router at set intervals. In this case, there has been no change in the topology, so no new information is contained in the updates. The animation is based on the network topology described in 4.2.1 diagram 1, except a timer clock is added next to each router.


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RIP Timers

In addition to the update timer, the IOS implements three additional timers for RIP:
  • Invalid
  • Flush
  • Holddown
Invalid Timer. If an update has not been received to refresh an existing route after 180 seconds (the default), the route is marked as invalid by setting the metric to 16. The route is retained in the routing table until the flush timer expires.

Flush Timer. By default, the flush timer is set for 240 seconds, which is 60 seconds longer than the invalid timer. When the flush timer expires, the route is removed from the routing table.

Holddown Timer. This timer stabilizes routing information and helps prevent routing loops during periods when the topology is converging on new information. Once a route is marked as unreachable, it must stay in holddown long enough for all routers in the topology to learn about the unreachable network. By default, the holddown timer is set for 180 seconds. The holddown timer is discussed in more detail later in this chapter.

Click show ip route in the figure.

The timer values can be verified with two commands: show ip route and show ip protocols. Notice in the output from show ip route that each route learned through RIP shows the elapsed time since the last update, expressed in seconds.

Click show ip protocols in the figure.

This information is also repeated in the show ip protocols output under the heading Last Update. The show ip protocols command details when this router, R1, is due to send out its next round of updates. It also lists the invalid, holddown, and flush timer default values.


4.3.1 - Periodic Updates: RIP v1 and IGRP
The diagram depicts verifying RIP timers using the show i p route and show i p protocols commands. The diagram is based on the network topology described in 4.2.1 diagram 1.

The output from the show i p route command displays each route learned through RIP, showing the elapsed time since the last update expressed in seconds. Partial output is highlighted in the diagram as follows:
R1#show i p route
output omitted
R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:04, Serial0/0/0
R 10.4.0.0 [120/2] via 10.2.0.2, 00:00:04, Serial0/0/0

The show i p protocols command displays details when router R1 is due to send out its next round of updates. It also lists the invalid, hold-down, and flush timer default values. Partial output is highlighted in the diagram as follows:

R1#show i p protocols
Routing Protocol is rip
Sending updates every 30 seconds, next due in 13 seconds
Invalid after 180 seconds, hold down 180, flushed after 240
output omitted

Routing for Networks:
10.0.0.0
Routing Information Sources:
Gateway: 10.3.0.1
Distance: 120
Last Update: 00:00:27

Distance: (default is 120)


4.3.2 Bounded Updates: EIGRP

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Unlike other distance vector routing protocols, EIGRP does not send periodic updates. Instead, EIGRP sends bounded updates about a route when a path changes or the metric for that route changes. When a new route becomes available or when a route needs to be removed, EIGRP sends an update only about that network instead of the entire table. This information is sent only to those routers that need it.

EIGRP uses updates that are:
  • Non-periodic because they are not sent out on a regular basis.
  • Partial updates sent only when there is a change in topology that influences routing information.
  • Bounded, meaning the propagation of partial updates are automatically bounded so that only those routers that need the information are updated.
Note: More details on how EIGRP operates will be presented in Chapter 9.


4.3.2 - Bounded Updates: EIGRP
The animation depicts EIGRP sending bounded updates. A new route to network 10.4.0.0 is sent by R1 only to select routers.


4.3.3 Triggered Updates

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To speed up the convergence when there is a topology change, RIP uses triggered updates. A triggered update is a routing table update that is sent immediately in response to a routing change. Triggered updates do not wait for update timers to expire. The detecting router immediately sends an update message to adjacent routers. The receiving routers, in turn, generate triggered updates that notify their neighbors of the change.

Triggered updates are sent when one of the following occurs:
  • An interface changes state (up or down)
  • A route has entered (or exited) the "unreachable" state
  • A route is installed in the routing table
Using only triggered updates would be sufficient if there were a guarantee that the wave of updates would reach every appropriate router immediately. However, there are two problems with triggered updates:
  • Packets containing the update message can be dropped or corrupted by some link in the network.
  • The triggered updates do not happen instantaneously. It is possible that a router that has not yet received the triggered update will issue a regular update at just the wrong time, causing the bad route to be reinserted in a neighbor that had already received the triggered update.
Play the animation to see how a network topology change is propagated through the network. When network 10.4.0.0 becomes unavailable and router 3 becomes aware of that, it sends out the information to its neighbors. The information is then propagated through the network.


4.3.3 - Triggered Updates
The animation depicts using triggered updates to speed up convergence following a topology change. When network 10.4.0.0 goes down, a triggered update is sent immediately from router R3. The receiving routers, R1 and R2, remove the route from their routing tables. The animation is based on the network topology described in 4.2.1 diagram 1.


4.3.4 Random Jitter

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Issues with Synchronized Updates

When multiple routers transmit routing updates at the same time on multi-access LAN segments (as shown in the animation), the update packets can collide and cause delays or consume too much bandwidth.

Note: Collisions are only an issue with hubs and not with switches.

Sending updates at the same time is known as the synchronization of updates. Synchronization can become a problem with distance vector routing protocols due to their usage of periodic updates. As more routers' timers become synchronized, more collisions of updates and more delays occur in the network. Initially, the updates of routers will not be synchronized. But over time, the timers across a network will become globally synchronized.

The Solution

To prevent the synchronization of updates between routers, the Cisco IOS uses a random variable, called RIP_JITTER, which subtracts a variable amount of time to the update interval for each router in the network. This random jitter, or variable amount of time, ranges from 0% to 15% of the specified update interval. In this way, the update interval varies randomly in a range from 25 to 30 seconds for the default 30-second interval.


4.3.4 - Random Jitter
The animation depicts synchronized updates. Over time, periodic routing updates can become synchronized. The animation is based on the following network topology.

Network Topology:
Six routers are shown, all connected to a hub. A synchronized update is sent from each router, resulting in collisions.


4.4 Routing Loops

4.4.1 Definition and Implications

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What is a Routing Loop?

A routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its intended destination network. A routing loop can occur when two or more routers have routing information that incorrectly indicates that a valid path to an unreachable destination exists.

The loop may be a result of:
  • Incorrectly configured static routes
  • Incorrectly configured route redistribution (redistribution is a process of handing the routing information from one routing protocol to another routing protocol and is discussed in CCNP-level courses)
  • Inconsistent routing tables not being updated due to slow convergence in a changing network
  • Incorrectly configured or installed discard routes
Distance vector routing protocols are simple in their operations. Their simplicity results in protocol drawbacks like routing loops. Routing loops are less of a problem with link-state routing protocols but can occur under certain circumstances.

Note: The IP protocol has its own mechanism to prevent the possibility of a packet traversing the network endlessly. IP has a Time-to-Live (TTL) field and its value is decremented by 1 at each router. If the TTL is zero, the router drops the packet.

What are the Implications of Routing Loops?

A routing loop can have a devastating effect on a network, resulting in degraded network performance or even a network downtime.

A routing loop can create the following conditions:
  • Link bandwidth will be used for traffic looping back and forth between the routers in a loop.
  • A router's CPU will be strained due to looping packets.
  • A router's CPU will be burdened with useless packet forwarding that will negatively impact the convergence of the network.
  • Routing updates may get lost or not be processed in a timely manner. These conditions would introduce additional routing loops, making the situation even worse.
  • Packets may get lost in "black holes."
Play the animation to view a possible routing loop scenario in which mechanisms to prevent such loops do not exist.

As you can see, routing loops eat up bandwidth and also router resources, resulting in a slow or even unresponsive network.

There are a number of mechanisms available to eliminate routing loops, primarily with distance vector routing protocols. These mechanisms include:
Triggered updates were discussed in the previous section. The other loop avoidance mechanisms are discussed later in this chapter.


4.4.1 - Definition and Implications
The animation depicts a routing loop, which can occur with distance vector routing protocols. The animation is based on the network topology described in 4.2.1 diagram 1.

In the animation, the 10.4.0.0 network that is directly connected to router R3 goes down. Before R3 can send an update, R2 sends an update, and R3 installs a bad route to 10.4.0.0. The network now has a loop.


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Use the Packet Tracer Activity to experience how a routing loop might occur with misconfigured static routes.

Click the Packet Tracer icon for more details.


4.4.1 - Definition and Implications
Link to Packet Tracer Exploration: Routing Loops

Use the Packet Tracer Activity to experience how a routing loop might occur with misconfigured static routes.


4.4.2 Problem: Count to Infinity

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Count to infinity is a condition that exists when inaccurate routing updates increase the metric value to "infinity" for a network that is no longer reachable. The animation shows what happens to the routing tables when all three routers continue to send inaccurate updates to each other.


4.4.2 - Problem: Count to Infinity
The animation depicts a condition called count to infinity. Count to infinity can occur when routing loops exist with distance vector routing protocols. The animation is based on the network topology described in 4.2.1 diagram 1.

In the animation, the 10.4.0.0 network that is directly connected to router R3 goes down. Before R3 can send an update, R2 sends an update. R3 installs a bad route to 10.4.0.0 with a hop count of 2. R3 sends an update to R2 with a hop count of 3 to 10.4.0.0. R2 increases the hop count to 3 for 10.4.0.0. R2 sends an update to R1 with a hop count of 4 to 10.4.0.0. R1 increases the hop count to 4 for 10.4.0.0. R2 sends the next periodic update with a hop count of 4 to 10.4.0.0. R3 increases the hop count to 4 for 10.4.0.0. Each round of updates continues to increase the hop count.


4.4.3 Setting a Maximum

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To eventually stop the incrementing of the metric, "infinity" is defined by setting a maximum metric value. For example, RIP defines infinity as 16 hops - an "unreachable" metric. Once the routers "count to infinity," they mark the route as unreachable.


4.4.3 - Setting a Maximum
The diagram depicts that 10.4.0.0 is now considered unreachable. The hop count is 16, and count to infinity is terminated. The animation is based on the network topology described in 4.4.1 diagram 1.


4.4.4 Preventing Routing Loops with Holddown Timers

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Earlier you learned that distance vector protocols employ triggered updates to speed up the convergence process. Remember that in addition to triggered updates, routers using distance vector routing protocols also send periodic updates. Let's imagine that a particular network is unstable. The interface resets as up, then down, then up again in rapid succession. The route is flapping. Using triggered updates, the routers might react too quickly and unknowingly create a routing loop. A routing loop could also be created by a periodic update that is sent by the routers during the instability. Holddown timers prevent routing loops from being created by these conditions. Holddown timers also help prevent the count to infinity condition.

Holddown timers are used to prevent regular update messages from inappropriately reinstating a route that may have gone bad. Holddown timers instruct routers to hold any changes that might affect routes for a specified period of time. If a route is identified as down or possibly down, any other information for that route containing the same status, or worse, is ignored for a predetermined amount of time (the holddown period). This means that routers will leave a route marked as unreachable in that state for a period of time that is long enough for updates to propagate the routing tables with the most current information.

Holddown timers work in the following way:

1. A router receives an update from a neighbor indicating that a network that previously was accessible is now no longer accessible.

2. The router marks the network as possibly down and starts the holddown timer.

3. If an update with a better metric for that network is received from any neighboring router during the holddown period, the network is reinstated and the holddown timer is removed.

4. If an update from any other neighbor is received during the holddown period with the same or worse metric for that network, that update is ignored. Thus, more time is allowed for the information about the change to be propagated.

5. Routers still forward packets to destination networks that are marked as possibly down. This allows the router to overcome any issues associated with intermittent connectivity. If the destination network truly is unavailable and the packets are forwarded, black hole routing is created and lasts until the holddown timer expires.

Play the animation to see an example of the holddown process.


4.4.4 - Preventing Routing Loops with Hold-down Timers
The animation depicts how using a hold-down timer can prevent routing loops and can help prevent the count to infinity condition. The animation is based on the network topology described in 4.4.1 diagram 1.

In the animation, the 10.4.0.0 network that is directly connected on router R3 goes down. A triggered update is sent by R3 to R2. R2 places 10.4.0.0 in hold down. R1 sends a periodic update. R2 sends a triggered update. R2 ignores the update from R1 about 10.4.0.0. R1 places 10.4.0.0 in hold down. R2 and R3 send out periodic updates. Traffic to 10.4.0.0 is still routed during the hold down. The hold-down timer on R2 expires, and 10.4.0.0 is removed from the routing table. R1 sends out a periodic update. The hold-down timer on R1 expires, and 10.4.0.0 is removed. The network is now converged.


4.4.5 Split Horizon Rule

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Another method used to prevent routing loops caused by slow convergence of a distance vector routing protocol is split horizon. The split horizon rule says that a router should not advertise a network through the interface from which the update came.

Applying split horizon to the previous example of route 10.4.0.0 produces the following actions:
  • R3 advertises the 10.4.0.0 network to R2.
  • R2 receives the information and updates its routing table.
  • R2 then advertises the 10.4.0.0 network to R1 out S0/0/0. R2 does not advertise 10.4.0.0 to R3 out S0/0/1, because the route originated from that interface.
  • R1 receives the information and updates its routing table.
  • Because of split horizon, R1 also does not advertise the information about network 10.4.0.0 back to R2.
Complete routing updates are exchanged, with the exception of routes that violate the split horizon rule. The results look like this:
  • R2 advertises networks 10.3.0.0 and 10.4.0.0 to R1.
  • R2 advertises networks 10.1.0.0 and 10.2.0.0 to R3.
  • R1 advertises network 10.1.0.0 to R2.
  • R3 advertises network 10.4.0.0 to R2.
Play the animation to see this process.

Notice that R2 sends different routing updates to R1 and R3.

Note: Split horizon can be disabled by an administrator. Under certain conditions, this has to be done to achieve the proper routing. These conditions are discussed in later courses.


4.4.5 - Split Horizon Rule
The animation depicts the split horizon rule for network 10.4.0.0, which helps prevent routing loops caused by slow convergence of a distance vector routing protocol. The animation is based on the network topology described in 4.4.1 diagram 1.

In the animation, R3 only advertises 10.4.0.0 to R2. R2 only advertises 10.3.0.0 and 10.4.0.0 to R1. R2 only advertises 10.2.0.0 and 10.1.0.0 to R3. R1 only advertises 10.1.0.0 to R2.


4.4.6 Split Horizon with Poison Reverse or Route Poisoning

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Route Poisoning

Route poisoning is yet another method employed by distance vector routing protocols to prevent routing loops. Route poisoning is used to mark the route as unreachable in a routing update that is sent to other routers. Unreachable is interpreted as a metric that is set to the maximum. For RIP, a poisoned route has a metric of 16.

Play the animation to see route poisoning in effect.
The following process occurs:
  • Network 10.4.0.0 becomes unavailable due to a link failure.
  • R3 poisons the metric with a value of 16 and then sends out a triggered update stating that 10.4.0.0 is unavailable.
  • R2 processes that update. Because the metric is 16, R2 invalidates the routing entry in its routing table.
  • R2 then sends the poison update to R1, indicating that route is unavailable, again by setting the metric value to 16.
  • R1 processes the update and invalidates the routing entry for 10.4.0.0 in its routing table.
Route poisoning speeds up the convergence process as the information about 10.4.0.0 spreads through the network more quickly than waiting for the hop count to reach "infinity".


4.4.6 - Split Horizon with Poison Reverse or Route Poisoning
The animation depicts route poisoning, which is another method employed by distance vector routing protocols to prevent routing loops. The animation is based on the network topology described in 4.4.1 diagram 1.

In the animation, the 10.4.0.0 network that is directly connected on router R3 goes down. R3 poisons the route with an infinite metric of 16. R3 sends a triggered poison update to R2. R2 poisons the route with an infinite metric of 16. R2 sends a triggered poison update to R1. R1 poisons the route with an infinite metric of 16. The network is converged on the poisoned route.


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Split Horizon with Poison Reverse

Poison reverse can be combined with the split horizon technique. The method is called split horizon with poison reverse. The rule for split horizon with poison reverse states when sending updates out a specific interface, designate any networks that were learned on that interface as unreachable.

The concept of split horizon with poison reverse is that explicitly telling a router to ignore a route is better than not telling it about the route in the first place.

Play the animation to see an example of the split horizon with poison reverse in effect.

The following process occurs:
  • Network 10.4.0.0 becomes unavailable due to a link failure.
  • R3 poisons the metric with a value of 16 and then sends out a triggered update stating that 10.4.0.0 is unavailable.
  • R2 processes that update, invalidates the routing entry in its routing table, and immediately sends a poison reverse back to R3.
Poison reverse is a specific circumstance that overrides split horizon. It occurs to ensure that R3 is not susceptible to incorrect updates about network 10.4.0.0.

Note: Split horizon is enabled by default. However split horizon with poison reverse may not be the default on all IOS implementations.


4.4.6 - Split Horizon with Poison Reverse or Route Poisoning
The animation depicts combining split horizon with poison reverse. Using this technique, when a router sends updates out a specific interface, it designates any networks that were learned on that interface as unreachable.

The animation is based on the network topology described in 4.4.1 diagram 1.

In the animation, the 10.4.0.0 network that is directly connected on router R3 goes down. R3 poisons the route with an infinite metric of 16. R3 sends a triggered poison update to R2 stating that the 10.4.0.0 route is unavailable. R2 processes that update and poisons the route with an infinite metric of 16. R2 sends a poison reverse back to R3.


4.4.7 IP and TTL

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Time to Live (TTL) is an 8-bit field in the IP header that limits the number of hops a packet can traverse through the network before it is discarded. The purpose of the TTL field is to avoid a situation in which an undeliverable packet keeps circulating on the network endlessly. With TTL, the 8-bit field is set with a value by the source device of the packet. The TTL is decreased by one by every router on the route to its destination. If the TTL field reaches zero before the packet arrives at its destination, the packet is discarded and the router sends an Internet Control Message Protocol (ICMP) error message back to the source of the IP packet.

The animation shows that even in the case of a routing loop packets will not loop endlessly in the network. Eventually the TTL value will be decreased to 0 and the packet will be discarded by the router.


4.4.7 - IP and TTL
The animation depicts the effects of the Time to Live (TTL) field in the IP header. TTL limits the number of hops a packet can traverse through the network before it is discarded. The animation is based on the network topology described in 4.4.1 diagram 1.

In the animation, the 10.4.0.0 network that is directly connected on router R3 goes down, and a routing loop is created for a packet. The packet TTL field begins with a value of 10 and is decreased by one each time the packet passes through each router, until it eventually reaches a value of zero, and the packet is discarded.


4.5 Distance Vector Routing Protocols today

4.5.1 RIP and EIGRP

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For distance vector routing protocols, there really are only two choices: RIP or EIGRP. The decision about which routing protocol to use in a given situation is influenced by a number of factors including:
  • Size of the network
  • Compatibility between models of routers
  • Administrative knowledge required
RIP

Over the years, RIP has evolved from a classful routing protocol (RIPv1) to a classless routing protocol (RIPv2). RIPv2 is a standardized routing protocol that works in a mixed vendor router environment. Routers made by different companies can communicate using RIP. It is one of the easiest routing protocols to configure, making it a good choice for small networks. However, RIPv2 still has limitations. Both RIPv1 and RIPv2 have a route metric that is based only on hop count and which is limited to 15 hops.

Features of RIP:
  • Supports split horizon and split horizon with poison reverse to prevent loops.
  • Is capable of load balancing up to six equal cost paths . The default is four equal cost paths.
RIPv2 introduced the following improvements to RIPv1:
  • Includes the subnet mask in the routing updates, making it a classless routing protocol.
  • Has authentication mechanism to secure routing table updates.
  • Supports variable length subnet mask (VLSM).
  • Uses multicast addresses instead of broadcast.
  • Supports manual route summarization.
EIGRP

Enhanced IGRP (EIGRP) was developed from IGRP, another distance vector protocol. EIGRP is a classless, distance vector routing protocol with features found in link-state routing protocols. However, unlike RIP or OSPF, EIGRP is a proprietary protocol developed by Cisco and only runs on Cisco routers.

EIGRP features include:
  • Triggered updates (EIGRP has no periodic updates).
  • Use of a topology table to maintain all the routes received from neighbors (not only the best paths).
  • Establishment of adjacencies with neighboring routers using the EIGRP hello protocol.
  • Support for VLSM and manual route summarization. These allow EIGRP to create hierarchically structured large networks.
Advantages of EIGRP:
  • Although routes are propagated in a distance vector manner, the metric is based on minimum bandwidth and cumulative delay of the path rather than hop count.
  • Fast convergence due to Diffusing Update Algorithm (DUAL) route calculation. DUAL allows the insertion of backup routes into the EIGRP topology table, which are used in case the primary route fails. Because it is a local procedure, the switchover to the backup route is immediate and does not involve the action in any other routers.
  • Bounded updates mean that EIGRP uses less bandwidth, especially in large networks with many routes.
  • EIGRP supports multiple Network layer protocols through Protocol Dependent Modules, which include support for IP, IPX, and AppleTalk.


4.5.1 - RIP and EIGRP
The diagram depicts an activity in which you replace each blank with a given term as it relates to the characteristics of the different distance vector routing protocols.

Terms for filling in the blanks (terms can be used more than once):
Slow
Fast
No
Yes
Low
Medium
High
Small
Large
Simple
Complex

Distance Vector Protocols and Characteristics:

RIP v1:
Speed of convergence: BLANK
Scalability - Size of network: BLANK
Use of VLSM: BLANK
Resource usage: BLANK
Implementation and maintenance: BLANK

RIP v2:
Speed of convergence: BLANK
Scalability - Size of network: BLANK
Use of VLSM: BLANK
Resource usage: BLANK
Implementation and maintenance: BLANK

IGRP:
Speed of convergence: BLANK
Scalability - Size of network: BLANK
Use of VLSM: BLANK
Resource usage: BLANK
Implementation and maintenance: BLANK

EIGRP:
Speed of convergence: BLANK
Scalability - Size of network: BLANK
Use of VLSM: BLANK
Resource usage: BLANK
Implementation and maintenance: BLANK


4.6 Lab Activities

4.6.1 Lab Activities

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In this lab activity, you recreate a network based only on the outputs from the show ip route command. Then, to verify your answer you configure the routers and check the actual routing table to the routing table shown in the lab documentation.

Click on the lab icon for more details.


4.6.1 - Lab Activities
Link to Hands-on Lab: Routing Table Interpretation Lab

In this lab activity, you recreate a network based only on the output from the show i p route command. To verify your answer, you configure the routers and check the actual routing table to the routing table shown in the lab documentation.


4.7 Summary

4.7.1 Summary and Review

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Summary

One way of classifying routing protocols is by the type of algorithm they use to determine the best path to a destination network. Routing protocols can be classified as distance vector, link state, or path vector. 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 routing protocols include:
  • RIPv1
  • RIPv2
  • IGRP
  • EIGRP
Routers that use distance vector routing protocols determine best path to remote networks based on the information they learn from their neighbors. If Router X learns of two paths to the same network, one through Router Y at 7 hops, and another route through Router Z at 10 hops, the router will choose the shorter path using Router Y as the next-hop router. Router X has no knowledge of what the network looks like beyond Routers Y and Z, and can only make its best path decision based on the information sent to it by these two routers. Distance vector routing protocols do not have a map of the topology as do link state routing protocols.

Network discovery is an important process of any routing protocol. Some distance vector routing protocols such as RIP go through a step-by-step process of learning and sharing routing information with their neighbors. As routes are learned from one neighbor, that information is passed on to other neighbors with an increase in the routing metric.

Routing protocols also need to maintain their routing tables to keep them current and accurate. RIP exchanges routing table information with its neighbors every 30 seconds. EIGRP, another distance vector routing protocol, does not send these periodic updates and only sends a "bounded" update when there is a change in the topology and only to those routers that need that information. EIGRP is discussed in a later chapter.

RIP also uses timers to determine when a neighboring router is no longer available, or when some of the routers may not have current routing information. This is typically because the network has not yet converged due to a recent change in the topology. Distance vector routing protocols also use triggered updates to help speed up convergence time.

One disadvantage of distance vector routing protocols is the potential for routing loops. Routing loops can occur when the network is in an unconverged state. Distance vector routing protocols use holddown timers to prevent the router from using another route to a recently down network until all of the routers have had enough time to learn about this change in the topology.

Split horizon and split horizon with poison reverse are also used by routers to help prevent routing loops. The split horizon rule states that a router should never advertise a route through the interface from which it learned that route. Split horizon with poison reverse means that it is better to explicitly state that this router does not have a route to this network by poisoning the route with a metric stating that the route is unreachable.

Distance vector routing protocols are sometime referred to as "routing by rumor", although this can be somewhat of a misnomer. Distance vector routing protocols are very popular with many network administrators as they are typically easily understood and simple to implement. This does not necessarily mean link-state routing protocols are any more complicated or difficult to configure. Unfortunately, link-state routing protocols have received this somewhat unwarranted reputation. We will learn in later chapters that link-state routing protocols are as easy to understand and configure as distance vector routing protocols.


4.7.1 - Summary and Review
In this chapter, you learned to:
- Identify the characteristics of distance vector routing protocols.
- Describe the network discovery process of distance vector routing protocols using Routing Information Protocol (RIP).
- Describe the processes for maintaining accurate routing tables that are used by distance vector routing protocols.
- Identify the conditions leading to a routing loop and explain the implications for router performance.
- Identify the types of distance vector routing protocols in use today.


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4.7.1 - Summary and Review
This is a review and is not a quiz. Questions and answers are provided.
Question 1. Briefly explain the basic operation of RIP and IGRP.
Answer: RIP and IGRP are distance vector routing protocols characterized by periodic updates that are broadcast to directly connected neighbors. The entire routing table is sent in the update.

Question 2. Explain convergence and why it is important.
Answer: Convergence is when all routers in the network have consistent and correct information about how to reach destination networks. A network is not completely operable until it has converged, so routing protocols require short convergence times.

Question 3. What are the four main timers in RIP? What is the purpose and duration of each?
Answer: Update timer (30 seconds) - Used to time when to send the next update.

Invalid timer (180 seconds) - Used to mark a route as invalid by setting the metric to 16 if an update has not been received to refresh an existing route after 180 seconds.

Hold-down timer (180 seconds) - The amount of time an unreachable route is in hold down.

Flush timer (240 seconds) - Time until a route is removed from the routing table.

Question 4. What five techniques do distance vector routing protocols use to prevent routing loops?
Answer:
- Defining the maximum metric to prevent count to infinity
- Hold-down timers
- Split horizon
- Route poisoning or poison reverse
- Triggered updates


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The Packet Tracer Skills Integration Challenge Activity for this chapter is very similar to the activity you completed at the end of Chapter 3. 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 implement an effective static routing configuration.

Packet Tracer Skills Integration Instructions (PDF)

Click the Packet Tracer icon for more details.


4.7.1 - 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 3. 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.


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To Learn More

Understanding the distance vector algorithm is not difficult. There are many book and online sources that show how algorithms such as the Bellman-Ford algorithm are used in networking. There are several web sites devoted to explaining how these algorithms work. Seek out some of the resources and familiarize yourself with how this algorithm works.

Here are some suggested resources:
  • Interconnections, Bridges, Routers, Switches, and Internetworking Protocols, by Radia Perlman
  • Cisco IP Routing, by Alex Zinin
  • Routing the Internet, by Christian Huitema


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


4.8 Quiz

4.8.1 Chapter Quiz

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4.8.1 - Chapter Quiz
1. Which four statements are true regarding distance vector routing protocols? (Choose four.)
A. Hop counts can be used for path selection.
B. They scale well.
C. Routing updates are broadcast at intervals.
D. EIGRP can do unequal-cost load balancing.
E. RIP v1 multicasts its routing updates.
F. RIP sends its entire routing table to directly connected neighbors.

2. Which three conditions cause distant vector routing protocols to send routing table updates? (Choose three.)
A. the hold-down timer expires
B. a change occurs in the network topology
C. the update timer value expires
D. a triggered update is received from another router
E. a packet is received that is destined for an unknown network
F. no routing table changes for 30 minutes

3. What are two characteristics of an EIGRP update? (Choose two.)
A. includes all EIGRP routes
B. includes the full routing table
C. independent of architecture
D. only triggered for route topology changes
E. broadcasted to affected neighbors
F. bounded to affected next hop routers

4. What feature was added to RIP to help with synchronization errors?
A. hold -down timer
B. RIP_JITTER
C. RIP_DELAY
D. jitter control

5. Which two timers are used for RIP? (Choose two.)
A. invalid
B. refresh
C. flush
D. deadlink
E. hello

6. Which statement is true concerning the advantages of a distance vector protocol?
A. Periodic updates speed up convergence.
B. Convergence times make routing loops impossible.
C. Ease of implementation makes configuration simple.
D. They work well in complex networks.
E. Their convergence times are faster than link-state routing protocols.

7. Which mechanism can be used to avoid a count to infinity loop?
A. split horizon
B. route poisoning
C. hold-down timer
D. triggered update
E. split horizon with poison reverse

8. Refer to the following topology description to answer the question.
Topology Description:
There are five routers, Router1, Router2, Router3, Router4, and Router5.
Router1 is connected to Router2 and Router4 via WAN links.
Router3 is connected to Router2 and Router4 via WAN links.
Router4 is connected to Router5.
Router5 has a LAN 10.0.0.0 attached.

Question: The network shown is running RIP. What mechanism will keep Router4 from sending updates about the 10.0.0.0 network back to Router5?
A. split horizon
B. poison reverse
C. route poisoning
D. hold-down timer
E. maximum hop count

9. What allows distance vector protocols to avoid routing loops by advertising a metric of infinity?
A. split horizon
B. route poisoning
C. hold-down timer
D. maximum hop count
E. time to live (TTL) field of IP header

10. Which field in the IP header ensures that packets do not loop endlessly on a network?
A. CRC
B. TOS
C. TTL
D. Checksum

11. Match the loop-preventing mechanism to the corresponding function.
Mechanisms:
1. split horizon
2. route poisoning
3. hold-down timer
4. triggered update

Functions:
A. Routes learned through an interface are not advertised out that same interface.
B. Routes learned through an interface are advertised back out the same interface as unreachable.
C. Topology changes are immediately sent to adjacent routers.
D. It allows time for topology changes to travel through the entire network.

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