Choosing Between Routing Protocols Using Administrative Distance

10 Mar

Choosing Between Routing Protocols Using Administrative Distance
Clearly, there are many IP routing protocols from which to choose. Choosing a single routing protocol is better, because the resulting consistency relates directly to the strength of the network. The network is complicated when more than a single routing protocol attempts to perform the same job.

When more than one routing protocol is running on the router, the routing process must make a decision to have one entry per prefix in the routing table. The choice cannot be based on the metric because metrics differ between routing protocols. Instead, another method, called administrative distance, was devised to solve the problem.

NOTE The routing table on a router running more than one routing protocol knows about all the networks heard by the various protocols and sends data to all of the distant networks, choosing the best path via administrative distance. However, a routing protocol only sends updates about networks it has knowledge of, so if IGRP knows about networks 1, 2, and 3, it propagates knowledge of these networks out of IGRP-configured interfaces to other IGRP routers on the same segment. It will not send out information about networks 4, 5, and 6 that were placed into the routing table by RIP. In order for the IGRP routers to hear of networks 4, 5, and 6, it is necessary to share the network information between the routing protocols. This is called redistribution. However, the router that is responsible for redistribution will have more than one process running, which takes extra resources.

The administrative distance selects one or more paths to enter the routing table from several paths offered by multiple routing protocols.

In Figure 4-2, for example, both RIP and EIGRP have paths to the network 140.100.6.0. RIP is configured on the FDDI ring and EIGRP is running on the rest of the network. On Router D, RIP is offering a metric of 2 hops, and EIGRP is offering a metric of 768. Without redistribution, no conversion or choice is possible, because there are no similar criteria for distinguishing the two paths. Therefore, the metric is ignored, and the administrative distance is used to make the selection. The administrative distance of EIGRP is lower than that of RIPv1, so the path advertised by EIGRP is chosen, despite the speed of Frame Relay set at 56 kbps as opposed to the 100 Mbps of FDDI. In this case, if it is not possible to run EIGRP on the FDDI ring because of proprietary restrictions, manually configuring the administrative distance on Router D would be advisable.

Choosing Between Routing Protocols Using Administrative Distance 147

Figure 4-2 Path Selection Using Administrative Distance

Routing Protocols

Administrative distance is a rather arbitrary set of values placed on the different sources of routing information. You can change the defaults, but proceed carefully when subverting the natural path selection. You must perform any manual configuration with careful reference to the network design of the organization and its traffic flow. The creation of floating static routes is an example of when the administrative distance is changed.

A lower administrative distance reflects the preferred choice. Table 4-2 lists the administrative distance defaults.

Table 4-2 Default Administrative Distance

Administrative Distance

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Table 4-2 Default Administrative Distance (Continued)

Administrative Distance

The administrative distance is looked at with total disregard of the metrics, which might result in a poor path selection. Problems can occur when redundancy is built into the network. For example, a low-cost, low-speed connection to a network can be used as a backup link to the core of the network or the segment that has the servers. The intention is for the link never to be used. The link is there as insurance against the primary link failing. Backup links for redundancy are often implemented over on-demand serial connections where the network charges are based on usage. However, in Figure 4-2, you have seen that this backup link would become the primary link.

To make this truly a backup link, you must configure it as a static route. However, the administrative distance of a static route takes precedence over everything but a directly connected network. Therefore, you need to configure manually the static route’s administrative distance to ensure that it takes precedence over any other route information only when the primary route fails. This design is called a floating static route.

Convergence
Convergence occurs when all the routers in the routing domain agree on the routes that are available. Convergence time is the time that it takes for every router’s routing table to synchronize after there has been a change in the network topology.

You need to ensure that the time taken is as short as possible, because while the routers disagree on the available networks, they cannot route data correctly or efficiently.

Each routing protocol has a different method of updating the routing table. This affects convergence time. The following sections introduce new concepts by explaining the different protocol convergence methods. The sections show the relative merits of each approach. The concepts are explained in depth in the chapters that concentrate on the specific protocols. The terms shown in italics are defined in the final glossary at the end of the book.

RIPv1 and RIPv2 Convergence
The steps for RIPv1 and RIPv2 convergence are as follows:
1. When the local router sees a connected route disappear, it sends a flash update and removes the route entry from its table. This is called a triggered update with poison reverse.
2. The receiving routers send flash updates and put the affected route in holddown.
3. The originating router queries its neighbor for alternative routes. If the neighbor has an alternative route, it is sent; otherwise, the poisoned route is sent.
4. The originating router installs the best alternative route that it hears because it has purged the original routes.
5. Routers that are in holddown ignore the alternative route.

When the other routers emerge from holddown, they will accept the alternative route.

Convergence takes the time for detection, plus holddown, plus the number of routing updates (equal to the hop-count diameter of the network).

IGRP Convergence
The steps for IGRP convergence are as follows:

1. When the local router sees a connected route disappear, it sends a flash update and removes the route entry from its table. This is called a triggered update with poison reverse.
2. The receiving routers send flash updates and put the affected route in holddown.
3. The originating router queries its neighbor for alternative routes. If the neighbor has an alternative route, it is sent; otherwise, the poisoned route is sent.
4. The originating router installs the best alternative route that it hears because it has purged the original routes. It sends a new flash update. This is the routing table, either with or without the network available, stating the higher metric.
5. Routers that are in holddown ignore the alternative route.
6. When the routers come out of holddown, they accept the alternative route.

When the other routers emerge from holddown, they will accept the alternative route.

Convergence takes the time for detection, plus holddown, plus the number of routing updates (equal to the hop-count diameter of the network). Because the time between updates is 90 seconds, this could take a very long time.

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EIGRP Convergence
The steps for EIGRP convergence are as follows:

1. When the local router sees a connected route disappear, it checks the topology table for a feasible successor. If no feasible successor exists, it moves into active state.
2. The originating router queries its neighbor for alternative routes, and the receiving router acknowledges.
3. If an alternative route exists, information about this route is sent to the querying router.
4. If the router receives an acceptable successor, it adds the route to the table.
5. The router sends out a flash update of the path with the higher metric.
6. The receiving router acknowledges the update.

Convergence is quick because it is the detection time plus query time, reply time, and update time. If there is a feasible successor, convergence is almost instantaneous.

Interior and Exterior Gateway Protocols
Routing protocols that operate within an organization are referred to as Interior Gateway Protocols (IGPs) or interior routing protocols (for example, RIPv1, RIPv2, IGRP, EIGRP, OSPF, and IS-IS).

The boundaries of the organization are defined as the autonomous system. The unique number assigned to the autonomous system then identifies the organization. The autonomous system number might be viewed as another layer of hierarchy in the IP addressing scheme, because the number can represent a collection of IANA numbers.

Routing protocols that exchange routing information between organizations are known as Exterior Gateway Protocols (EGPs). EGPs are highly complex. The complexity arises from the need to determine policies between different organizations. Border Gateway Protocol Version 4 (BGP-4) is the only current example of an EGP.

Foundation Summary

The “Foundation Summary” section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a wellprepared candidate should, at a minimum, know all the details in each “Foundation Summary” before going to take the exam.

The following characteristics describe not only RIPv1, but in essence, any distance vector routing protocol.

■ Count to infinity —A router (A) hears about networks from its neighbors (B and C) and updates the routing table with the new networks. The routing table is then sent to all neighbors (B and C). However, if a neighbor (B) is sent information about networks for which it was the originating source, confusion can occur, referred to as a routing loop. The problem occurs when the path to a network goes down; each router might believe that there is an alternative path through its neighbor.

The ramifications of this problem are limited because each router increments the hop count before it sends out the update. When the hop count reaches 16, the network is
rejected as unreachable because the diameter of a RIPv1 network cannot be greater than 15. This is called counting to infinity, where “infinity” equals 16. Although the liability is controlled, it will still slow convergence of the network.

■ Split horizon —This is a mechanism to prevent loops. If split horizon works, the need for “count to infinity” is eliminated. The split horizon rule states that the routing process will not advertise networks out of the interface through which those networks were heard. This prevents information about networks being repeated to the source of those networks.
■ Split horizon with poison reverse —Split horizon on its own might not prevent loops, though it prevents networks being advertised out of the interface from which they were learned. However, poison reverse overrides split horizon when a network is lost. Poison reverse includes all the networks that have been learned from the neighbor, but it sets the metric to infinity (16). By changing the metric value to 16, the networks are reported to be unreachable. The routing process acknowledges the network but denies a valid path. Although this increases network overhead by increasing the update size, split horizon with poison reverse can prevent loops.
■ Holddown —After deciding that a network in the routing table is no longer valid, the routing process waits for three routing updates (by default) before it believes a routing update with a less-favorable metric. Again, this is to prevent routing loops from generating false information throughout the network.

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■ Triggered updates —As soon as a routing process changes a metric for a network in its routing table, it sends an update with the metric set to a value that states it is unusable. In RIP, this value is infinity, that is, 16. Triggered updates inform the other routers immediately. If there is a problem in the network, all the affected routers go into holddown immediately instead of waiting for the periodic timer. This mechanism increases convergence and helps prevent loops.
■ Load balancing —If the routing process sees multiple paths of equal cost to a remote network, it distributes the routed (datagram) traffic evenly among the paths. It will allocate datagrams to the different paths on a round-robin basis. The type of switching that is used—process switching or fast switching—will determine whether the load balancing is done on a roundrobin or session basis. Round-robin load balancing is used when there is process switching in effect.

Table 4-3 summarizes default administrative distances.
Table 4-3 Default Administrative Distance

summarizes default administrative distances

Q&A
As mentioned in the introduction, “All About the CCNP, CCDP, and CCIP Certifications,” you have two choices for review questions. The questions that follow give you a bigger challenge than the exam itself by using an open-ended question format. By reviewing now with this more difficult question format, you can exercise your memory better and prove your conceptual and factual knowledge of this chapter. The answers to these questions are found in Appendix A.

For more practice with examlike question formats, including questions using a router simulator and multichoice questions, use the exam engine on the CD-ROM.

1. Name one routing protocol that sends periodic updates.
2. What is an incremental update, and how often is it sent out?
3. Distance vector routing protocols naturally summarize at which boundary?
4. What is the algorithm used by distance vector protocols?
5. Give three reasons why RIPv1 has problems working in a large network.
6. What is the destination address of the distance vector periodic update in RIPv1?
7. State two ways that a route is selected as the preferred path.
8. What is administrative distance?
9. If IGRP has three paths to a remote network in which each path has an equal metric, what will happen?
10. A distance vector routing protocol uses the mechanism of poison reverse. What is poison reverse?
11. Name two distance vector routing protocols.
12. Describe the mechanism of split horizon.
13. What is meant by the phrase routing by rumor?
14. Why does the use of multicast addressing in RIPv2 overcome some of the limitations of RIPv1?
15. Explain the use of holddown in distance vector routing protocols to create stability in the network.
16. What is the maximum hop count in RIPv1 and RIPv2?
17. Both EIGRP and IGRP use a composite metric. What are the main components of this metric?
18. Explain briefly how RIPv2 differs from RIPv1.
19. What is meant by the term convergence?
20. Give the configuration commands to turn on the process for RIPv2.

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