236BGP configurationThe BGP is a dynamic inter-AS exterior gateway protocol.The term router refers to both routers and Layer 3 switches, and BGP refers to BGP-4 in this document.BGP overviewThere are three early BGP versions: BGP-1 (RFC 1105), BGP-2 (RFC 1163), and BGP-3 (RFC1267). The current version in use is BGP-4 (RFC 4271), which is the defacto Internet exteriorgateway protocol used between ISPs.The characteristics of BGP are as follows:• Focusing on the control of route propagation and the selection of optimal routes rather thanthe route discovery and calculation, which makes BGP, an exterior gateway protocoldifferent from interior gateway protocols such as OSPF and RIP• Using TCP to enhance reliability• Supporting CIDR• Reducing bandwidth consumption by advertising only incremental updates and, thereforeapplicable to advertising a great amount of routing information on the Internet• Eliminating routing loops by adding AS-PATH information to BGP routes• Providing abundant policies to implement flexible route filtering and selection• Good scalabilityA router advertising BGP messages is called a BGP speaker, as it establishes peer relationshipswith other BGP speakers to exchange routing information. When a BGP speaker receives a newroute, or a route better than the current one from another AS, it will advertise the route to all theother BGP peers in the local AS.To simplify configuration, multiple peers having an identical policy can be organized as a peergroup.BGP runs on a router in one of the following modes:• iBGP• eBGPBGP is called iBGP when it runs within an AS, and eBGP when it runs between ASs.
