A standard TCP/IP protocol based on the distance vector routing algorithm that enables groups of routers to share their routing information in an efficient manner. Border Gateway Protocol (BGP) version 4 is used to connect backbone routers on the Internet and is implemented by many Internet service providers (ISPs). BGP has largely superseded the earlier Exterior Gateway Protocol (EGP).
BGP works by listening to traffic being routed and inferring information concerning the reachability of neighboring networks. BGP supports policy-based routing, which enables network traffic to be routed differently according to its cost, importance, or security needs.
BGP divides an internetwork into groups of routers (called autonomous systems) that have trusted routes between them. This limits the size of the routing table in order to optimize reliability and performance on large internetworks such as the Internet.
The first update of a router’s BGP routing table includes all known routes on the network, while succeeding updates are only incremental. BGP also supports classless interdomain routing (CIDR). BGP is defined in Request for Comments (RFC) 1771 through 1774.