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BGP is a TCP/IP-based exterior routing protocol that is used between autonomous systems (ASs), primarily on the Internet. An AS is basically an Internet service provider in this context, although it defines any group of networks managed by a central authority. ASs are also referred to as domains.

In contrast, Interior routing protocols, such as OSPF (Open Shortest Path First), operate inside autonomous systems. Refer to RFC 1403 (BGP OSPF Interaction, January 1993) for information about interfacing between BGP and OSPF.

BGP runs in the routers that are connected to other autonomous systems. Those routers are called boundary routers. They are truly gateways because they provide connections to networks outside of the autonomous system. BGP exchanges routing and reachability information over these links.

Currently, BGP is in version 4, which supports CIDR (Classless Inter-domain Routing). Refer to "Routing on the Internet" for a historical perspective on the development of Internet routing, including information about the earlier EGP protocol.

RFC 1771 (Border Gateway Protocol 4, March 1995) describes the latest version of BGP. Also see RFC 2519 (A Framework for Inter-Domain Route Aggregation, February 1999) for more information. For a complete list of related RFCs, refer to the IETF Inter-Domain Routing (idr) Working Group Web site listed on the related entries page.