Chapter 11. Implementing Redistribution and Controlling Routing Updates

This chapter covers the following topics:

  • Understanding Redistribution Fundamentals—Describes reasons why multiple protocols may exist within an organization and introduces the basics of redistribution.

  • Understanding the Routing Decisions that Affect Redistribution—Describes routing metrics, path selection, and potential problems associated with redistribution.

  • Controlling Routing Updates During Redistribution—Describes the methods to control the routing information sent between routers during redistribution.

  • Configuring Redistribution—Describes the steps involved in setting up redistribution.

  • Controlling Routing Updates with Filtering—Describes the use of distribute lists.

  • Verifying, Maintaining, and Troubleshooting the Redistribution and Filtering Implementation—Focuses on the traceroute and extended ping commands.

The topics in this chapter deal with the traffic generated by routing updates in terms of both the network resources that they use and the information contained within them. The chapter covers two different but related areas, redistribution and filtering. The network overhead involved in routing updates has already been dealt with in other chapters; it keeps recurring as a theme because all network traffic directly influences the network's capability to scale or to grow.

The information propagated through the network is complex when dealing with one routing protocol. When multiple protocols have to share information (through redistribution) so that the larger network can see every route available within the autonomous system (AS), the information flow must be controlled and closely managed with filtering and other solutions.

This chapter deals with the need for redistribution, which increases the network overhead, and filtering, which is used to reduce overhead. The chapter first explains the design issues that might affect the configuration, and then provides configuration examples of route redistribution and filtering.

The purpose of the "Do I Know This Already?" quiz is to help you decide what parts of this chapter to use. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.

The 16-question quiz, derived from the major sections in the "Foundation Topics" portion of the chapter, helps you determine how to spend your limited study time.

Table 11-1 outlines the major topics discussed in this chapter and the "Do I Know This Already?" quiz questions that correspond to those topics.

Table 11-1. "Do I Know This Already?" Foundation Topics Section-to-Question Mapping
Foundation Topics SectionQuestions Covered in This SectionScore
Understanding Redistribution Fundamentals1–3 
Understanding the Routing Decisions That Affect Redistribution4–7 
Controlling Routing Updates During Redistribution8–9 
Configuring Redistribution10–11 
Controlling Routing Updates with Filtering12–14 
Verifying, Maintaining, and Troubleshooting the Redistribution and Filtering Implementation15–16 
Total Score  


The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark this question wrong for purposes of the self-assessment. Giving yourself credit for an answer you correctly guess skews your self-assessment results and might provide you with a false sense of security.

1.Which of the following are reasons to run multiple routing protocols?
  1. Applications requiring UNIX

  2. Smaller routing domains speed up convergence

  3. Transitioning from one routing protocol to another

  4. Smaller routing domains ensure a more stable network

2.EIGRP automatically redistributes into which routing protocols?
  1. IGRP running the same AS number

  2. IGRP with any process ID

  3. EIGRP running the same AS number

  4. EIGRP with any process ID

3.How many IP routing tables can be held on a router within a typical organization?
  1. One

  2. One per routing protocol

  3. Four

  4. Six

4.The problems experienced as a result of multiple routing processes and their redistribution include which of the following?
  1. Suboptimal path

  2. Loss of hello packets

  3. Routing loops

  4. Continuous LSA propagation

5.Which command is used to configure the administrative distance?
  1. administrative distance

  2. distance

  3. ip default-distance

  4. ip administrative distance

6.What action is taken if no seed or default metric is configured for OSPF when redistributing EIGRP?
  1. The route is not entered into the routing table.

  2. The route is entered with a cost of 0.

  3. The route is read into OSPF with a cost of 20 (type E2).

  4. The route is entered with a cost of 20 (type 1).

7.What techniques can be employed to avoid redistribution problems?
  1. Distribute lists

  2. Change administrative distance

  3. Ensure the default metric is set to 0

  4. Redistribute on all border routers in both directions

8.Which of the following may be necessary to control the routing updates?
  1. Security

  2. Prevention of routing loops

  3. Scaling the network

  4. Preserving the metric

9.Which of the following is a complex access list that permits conditional programming?
  1. distribute list

  2. passive interface

  3. route map

  4. filter list

10.Which command is used to establish the default or seed metric for EIGRP?
  1. default-metric 5

  2. metric bandwidth delay reliability loading mtu

  3. default-metric bandwidth delay reliability loading mtu eigrp

  4. default-metric bandwidth delay reliability loading mtu

11.What is the default metric type value for routes redistributed into OSPF?
  1. 1

  2. 2

  3. 3

  4. 4

12.What is the purpose of distribute lists?
  1. Determine the administrative distance of a distributed routing protocol

  2. Identify which interfaces will send updates

  3. Determine which networks are sent in updates

  4. Determine which networks are accepted into the routing table

13.Where are distribute lists defined?
  1. At the interface

  2. Under the routing process

  3. At the router level

  4. At the executive prompt

14.All the following statements are true; which of the following actions is taken first?
  1. Do not advertise the route if it is matched by a deny statement.

  2. If no match is found in the distribute list, the implicit deny any at the end of the access list will cause the update to be dropped.

  3. If a filter is present, the router examines the access list to see if there is a match on any of the networks in the routing update.

  4. Advertise the route if matched by a permit statement.

15.Which commands could be used to verify and troubleshoot a network that is redistributed?
  1. show ip protocol

  2. show ip route

  3. show ip route routing-protocol

  4. show redistributed

16.Which of the following commands is not useful for tracking redistribution problems?
  1. show ip protocol

  2. show ip route

  3. show ip ospf database

  4. show ip eigrp neighbors

  5. show ip interface brief

The answers to this quiz are found in Appendix A, "Answers to Chapter 'Do I Know This Already?' Quizzes and Q&A Sections." The suggested choices for your next step are as follows:

  • 9 or less overall score—Read the entire chapter. This includes the "Foundation Topics," "Foundation Summary," "Q&A," and "Scenarios" sections.

  • 10–13 overall score—Begin with the "Foundation Summary" section, and then go to the "Q&A" and "Scenarios" sections. If you have trouble with these exercises, read the appropriate sections in "Foundation Topics."

  • 14–16 overall score—If you want more review on these topics, skip to the "Foundation Summary" section, and then go to the "Q&A" and "Scenarios" sections at the end of the chapter. Otherwise, move to the next chapter.