Border Gateway Protocol (BGP) is a routing protocol used on the edge of autonomous systems (AS). It is an exterior routing protocol and calculates loop-free paths across the Internet. It is considered to use a path-vector routing algorithm. This means it tracks the path in terms of which AS it passes through, and does NOT track the 'route' through individual routers within an AS, and is not specifically capable of performing load balancing or packet forwarding itself. BGP is the routing protocol of choice and is used by all the Network Service Providers (NSPs) such as UUNet, Sprint, Cable & Wireless, Level3, Qwest etc. It is dynamic and handles outages and link failures fairly gracefully. To use BGP, you must have a router that supports BGP; register an AS Number and contact your provider to set up a BGP session. See the requirements page for more information.
BGP has gone through three revisions. The current version in use is BGP4 and is supported by most router manufacturers including Cisco, Lucent/Bay, Juniper and many others, as well as by Unix and Linux programs such as Zebra.
BGP uses a TCP connection to send routing updates using TCP port 179. BGP is therefore by definition a 'reliable' protocol. While BGP version 3 provides for the dynamic learning of routes, BGP 4 adds additional route dampening functionality, communities, MD5 and multicasting capability.