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Configuring BGP P1C-145Configuring BGPThis chapter describes how to configure Border Gateway Protocol (BGP). For a completedescription of the BGP commands in this chapter, refer to the “BGP Commands” chapter of theNetwork Protocols Command Reference, Part 1. To locate documentation of other commands thatappear in this chapter, use the command reference master index or search online.The Border Gateway Protocol, as defined in RFCs 1163 and 1267, is an Exterior Gateway Protocol(EGP). It allows you to set up an interdomain routing system that automatically guarantees theloop-free exchange of routing information between autonomous systems.For protocol-independent features, see the chapter “Configuring IP Routing Protocol-IndependentFeatures” in this document.Cisco’s BGP ImplementationIn BGP, each route consists ofa network number, a list ofautonomous systems that information haspassed through (called the autonomous system path), and a list ofother path attributes. We supportBGP Versions 2, 3, and 4, as defined in RFCs 1163, 1267, and 1771, respectively.The primary function of a BGP system is to exchange network reachability information with otherBGP systems, including information about the list of autonomous system paths. This informationcan be used to construct a graph of autonomous system connectivity from which routing loops canbe pruned and with which autonomous system-level policy decisions can be enforced.You can configure the value for the Multi Exit Discriminator (MED) metric attribute using routemaps. (The name ofthis metric forBGPVersions 2 and3 is INTER_AS_METRIC.) When an updateis sent to an IBGP peer, the MED is passed along without any change. This action enables all thepeers in the same autonomous system to make a consistent path selection.A third-party next-hop router address is used in the NEXT_HOP attribute, regardless of theautonomous system of that third-party router. The Cisco IOS software automatically calculates thevalue for this attribute.Transitive, optional path attributes are passed along to other BGP-speaking routers.BGP Version 4 supports classless interdomain routing (CIDR), which lets you reduce the size ofyourrouting tables by creating aggregate routes, resulting insupernets. CIDReliminates the conceptof network classes within BGP and supports the advertising of IP prefixes. CIDR routes can becarried by OSPF, Enhanced IGRP, and ISIS-IP, and RIP.See the “BGP Route Map Examples” section at the end of this chapter for examples of how to useroute maps to redistribute BGP Version 4 routes.