Chapter 16. Controlling BGP Route Selection

This chapter covers the following topics:

BGP Attributes—Describes the BGP attributes used for determining BGP behavior.
Controlling BGP Path Selection—Describes explains how to configure BGP attributes on Cisco routers.
Verifying the Configuration of Attributes—Describes Cisco IOS commands to display output related to BGP attributes.

The topics in this chapter describe the values and process used in BGP path selection and build on the basic configuration of BGP described in the previous chapter. In this discussion of advanced BGP topics, explanation of the technology is coupled with configuration examples so that your conceptual understanding of BGP is reinforced by concrete implementation examples.

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

Table 16-1 outlines the major topics discussed in this chapter and the "Do I Know This Already?" quiz questions that correspond to those topics.

Table 16-1. "Do I Know This Already?" Foundation Topics Section-to-Question Mapping
Foundation Topics Section	Questions Covered in This Section	Score
BGP Attributes	1–5
Controlling BGP Path Selection	6–12
Verifying the Configuration of Attributes	13–14
Total Score

Caution

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.	Well-known mandatory attributes are described as which of the following? Must be included in updates Must be supported Stripped if not recognized Marked partial if not recognized
2.	Optional nontransitive attributes are described as which of the following? Must be included in updates Must be supported Stripped if not recognized Marked partial if not recognized
3.	Which BGP attributes are used in BGP path selection? AS-Path Atomic Aggregate Community Next Hop Origin
4.	A route has an AS-Path of 7 25 19 69. What can you learn from reading this path? The local AS is 7. The origin AS is 69. There is a loop. 7 is the origin.
5.	Place the following attributes in the order that BGP evaluates them to select a best path: AS-Path, Local Preference, MED, Weight. AS-Path, Local-Preference, MED, Weight. Local-Preference, AS-Path, Weight, MED. AS-Path, MED, Weight, Local-Preference. Weight, Local-Preference, AS-Path, MED.

6.	Give the command that would change the weight attribute for the path to the peer 192.10.5.3. set weight 192.10.5.3 48 neighbor 192.10.5.3 weight 48 set attribute weight 48 ip bgp neighbor 192.10.0.0 remote-as 300 weight 48
7.	Which of the following statements are true about the BGP weight attribute? Default value is zero BGP prefers the lowest value It is proprietary Only used within an AS
8.	Which of the following statements about the BGP local-preference attribute are true? Default value is zero BGP prefers the highest value It is proprietary Only used outside an AS
9.	Which of the following statements describes MED? Default value is 1 Only applies to external neighbors BGP prefers higher values It is a "strong" attribute
10.	Consider the following route map. What will be the local-preference for 192.168.26.0/24? access-list 39 permit 192.168.24.0 0.0.3.255 access-list 32 permit 192.168.26.0 0.0.0.255 route-map test permit 10 match ip address 39 match ip address 32 set ip local preference 150 route-map test permit 20 match ip address 39 32 set ip local preference 200 0 100 150 200

11.	Consider the route map from the previous question. What will be the local-preference for 192.168.25.0/24? 0 100 150 200
12.	Consider the route map from the previous question. What will be the local-preference for 192.168.20.0/24? 0 100 150 200
13.	In the command show ip bgp, the "i" origin code is placed at the end of a line in the BGP table. It can be which of the following values? Entry originated from IGP and was advertised with a network router configuration command. Entry originated from an EGP. Origin of the path is not clear. Usually, this is a router that is redistributed into BGP from an IGP. The IP address of the originating router.
14.	Which command shows the local preference and weight attribute values? show bgp attributes show ip bgp show ip bgp path show ip bgp attributes

The answers to the "Do I Know This Already?" 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:

10 or less overall score—Read the entire chapter. This includes the "Foundation Topics," "Foundation Summary," "Q&A," and "Scenarios" sections.
11 to 12 overall score—Begin with the "Foundation Summary" section, and then go to the "Q&A" and "Scenarios" sections at the end of the chapter. If you have trouble with these exercises, read the appropriate sections in "Foundation Topics."
13 or 14 overall score—If you want more review on these topics, skip to the "Foundation Summary" section, and then go to the "Q&A" section and "Scenarios" sections at the end of the chapter. Otherwise, move on to the next chapter.