Chapter 3. Configuring and Troubleshooting Border Gateway Protocol 4

• Basic BGP Configuration— This section provides a series of case studies for BGP configuration, including peering BGP routers, injecting IBGP routes into BGP, injecting BGP routes into an IGP, IBGP without an IGP, IBGP over an IGP, EBGP multihop, and aggregate routes.

• Managing BGP Connections— This section examines a variety of commands and tools that are available for making BGP connections more manageable from both an administrative and a maintenance standpoint.

• Routing Policies— This section discusses resetting BGP connections and provides a series of case studies covering filtering routes by network layer reachability information (NLRI), by AS_PATH, and with route maps; administrative weight; administering distances and backdoor routes; using the LOCAL_PREF and MULTI_EXIT_DISC attributes; prepending the AS_PATH; route tagging; and route dampening.

• Large-Scale BGP— This section provides a series of case studies for large-scale BGP design, including BGP peer groups, BGP communities, private AS numbers, BGP confederations, and route reflectors.

Many newcomers to BGP approach the protocol with trepidation. The source of this sentiment is the fact that BGP implementations are much more rare than IGP implementations. Outside of ISPs, most network administrators deal with BGP far less than with IGPs, if at all. Even when BGP is used, the configurations in small ISPs and non-ISP subscribers are usually pretty basic. Because most networking professionals lack in-depth experience with the protocol, it is often viewed as mysterious or intimidating.

You learned in Chapter 2, "Introduction to Border Gateway Protocol 4," that BGP itself is a relatively simple protocol. Certainly it is less complex than EIGRP, OSPF, or Integrated IS-IS. The complexity of BGP is not in the protocol, but in the scenarios in which it is used and the powerful tools associated with it. If an AS is not multihomed, or has only basic routing policies, BGP is usually unnecessary.

This chapter begins with basic BGP configurations and then presents some examples of using BGP to set routing policies—rules for sending and receiving route advertisements. Configuring BGP in large autonomous systems is covered last.

The configuration options available to BGP are so numerous that troubleshooting cannot be demonstrated adequately in just a few case studies. Therefore, this chapter presents troubleshooting issues in parallel with many configuration options and cases.