Summary

BGP was born out of the requirement to move away from a backbone-centered tree-topology and into a more distributed-backbone, autonomous-system based topology. BGP version 4 (BGP4) makes it possible to ensure reliable routing that enables route aggregation, multihoming, and policy-based routing.

BGP provides support for multihoming across up to eight paths to a destination. BGP's flexibility and complexity lie in its use of attributes (carried by community values) in route maps to "customize" a network topology. The attributes as defined by RFC 1771 are listed here:

• ORIGIN

• AS_PATH

• NEXT_HOP

• MULTI_EXIT_DESC (MED)

• LOCAL_PREF

• ATROMIC_AGGREGATE

• AGGREGATOR

BGP develops a network topology map of connectivity, exchanging network reachability information with other BGP systems, using this information to prevent routing loops. BGP uses route, path, and community filtering to control the flow of BGP updates between BGP autonomous systems (ASes). BGP provides a scalable and flexible Internet routing protocol to provide for loop-free routing information and connectivity between Internet Service Providers and customers and for large enterprise networks.

Network reachability information between BGP systems is accomplished with the use of route aggregation, BGP confederations, and route reflectors.

BGP does not use administrative distances during the route selection process. Instead, BGP uses administrative distances to select the routes that are placed in the routing table.

Three administrative distances are associated with BGP:

• Internal (IBGP) Routes are learned from IBGP peers. Default administrative distance is 200.

• External (EBGP) Routes are learned from EBGP peers. Default administrative distance is 20.

• Local Routes are generated locally. Default administrative distance is 20.

BGP is a very complex routing protocol, about which volumes of books have been published. Here is some recommended reading if you want to learn more about BGP:

• Internet Routing Architectures, Second Edition. Sam Halabi (published by Cisco Press, 2000).

• Cisco BGP-4 Command and Configuration Handbook. William R. Parkhurst, PhD (published by Cisco Press, 2001).

• Routing in the Internet, Second Edition. Christian Huitema (published by Prentice Hall, 2000).

• RFC 1771 A Border Gateway Protocol 4 (BGP-4). Y. Rekhter, T. Li. March 1995.