Border Gateway Protocol

The Border Gateway Protocol (BGP) is the core routing protocol of the Internet It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems(AS). It is described as a path vector protocol BGP does not use traditional IGP metrics, but makes routing decisions based on path, network policies and/or rulesets.
BGP was created to replace the EGP routing protocol to allow fully decentralized routing in order to allow the removal of the NSFNet Internet backbone network. This allowed the Internet to become a truly decentralized system. Since 1994, version four of the protocol has been in use on the Internet. All previous versions are now obsolete. The major enhancement in version 4 was support of Classless Inter-Domain Routing and use of route aggregation to decrease the size of routing tables.
Most Internet users do not use BGP directly. However, since most Internet service providers must use BGP to establish routing between one another (especially if they are multihomed it is one of the most important protocols of the Internet. which is the inter-provider core call setup protocol on the PSTN Very large private IP networks use BGP internally, however. An example would be the joining of a number of large Open Shortest Path First (OSPF) networks where OSPF by itself would not scale to size. Another reason to use BGP is multihoming a network for better redundancy either to multiple access points of a single ISP (RFC 1998) or to multiple ISPs.