BGP

Border Gateway Protocol (BGP) is a standardized designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. The protocol is often classified as a but is sometimes also classed as a . The Border Gateway Protocol makes routing decisions based on paths, network policies, or rule-sets configured by a network administrator and is involved in making core routing decisions.

BGP may be used for routing within an autonomous system. In this application it is referred to as Interior Border Gateway Protocol, Internal BGP, or iBGP. In contrast, the Internet application of the protocol may be referred to as Exterior Border Gateway Protocol, External BGP, or eBGP.

The current version of BGP is version 4 (BGP4), which was published as RFC 4271 in 2006, after progressing through 20 drafts documents based on RFC 1771 version 4. RFC 4271 corrected errors, clarified ambiguities, and updated the specification with common industry practices. The major enhancement was the support for Classless Inter-Domain Routing and use of route aggregation to decrease the size of routing tables. BGP4 has been in use on the Internet since 1994.

BGP4 is standard for Internet routing, required of most Internet service providers (ISPs) to establish routing between one another. Very large private networks use BGP internally. An example is the joining of a number of large Open Shortest Path First (OSPF) networks, when OSPF by itself does not scale to the size required. Another reason to use BGP is multihoming a network for better redundancy, either to multiple access points of a single ISP or to multiple ISPs.

BGP neighbors, called peers, are established by manual configuration between routers to create a session on port 179. A BGP speaker sends 19-byte keep-alive messages every 60 seconds to maintain the connection. Among routing protocols, BGP is unique in using TCP as its transport protocol.

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