Chapter 11. Configuring Basic BGP Functions and Attributes

This chapter covers the following key topics:

  • Building peering sessions— Configuration examples for the first step in the routing task. This section covers basic syntax used in configuration code.

  • Route filtering and attribute manipulation— BGP route maps, filtering based on NLRI, and filtering based on AS_PATH.

  • Peer groups— Configuration examples of defining and utilizing peer groups.

  • Sources of routing updates— Dynamic and static configuration for injecting information into BGP.

  • Overlapping protocols (backdoors)— Configuration examples for changing the distance parameter to favor certain routes over others.

  • BGP attributes— Configuration examples for NEXT_HOP, AS_PATH, local preference, MED, and community attributes.

  • BGP-4 aggregation— Configuration examples for various aggregation scenarios.

This is the first of two chapters consisting primarily of configuration examples. Having covered all the important prerequisite concepts, you can delve into these examples of how to write code for basic BGP functions and attributes. This chapter focuses on those basics, and the next chapter considers some of the more complex design-oriented configuration problems.

Even if you have been using the references in previous chapters to flip ahead to these configuration examples, you are encouraged to reexamine them now, with the benefit of having read and assimilated all the concept-oriented chapters. In addition to the configuration code itself, be sure to look at the many routing tables that are included; they are intended to solidify your understanding of what results to expect.

Chapters 11 and 12 are not intended to replace Cisco manuals and do not cover every command and scenario. They present configurations for common situations that are encountered in connecting networks to the Internet. Your particular network might require a combination of scenarios—or a different approach—to achieve the most effective policies.

In the following discussions, an AS could play the role of a customer, provider, or both. Do not get confused by having AS numbers and AS roles switched around, or by IP address numbering not being too realistic. These are just exercises that will help you understand BGP so that you can apply it accordingly in your own environment.

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