The purpose of an exterior gateway protocol is to enable two different Autonomous System (AS) to exchange routing information so that IP traffic can flow across the autonomous system border.

BGP was developed for use in conjunction with internets that employ the TCP/IP protocol suite. The BGP is an interdomain routing protocol that is used to exchange network reachability information among BGP routers (Also called BGP speakers).

Each BGP speaker establishes a TCP connection with one or more BGP speakers (routers). Two routers are considered to be neighbours if they are attached to the same subnetwork. If the two routers are in different autonomous systems, they may wish to exchange routing information.

BGP performs three functional procedures.

1. Neighbour acquisition

2. Neighbour reachability

3. Network reachability

Neighbour acquisition procedures used for exchanging the routing information between two routers in different autonomous systems (AS). To perform neighbour acquisition, one router sends an open message to another. If the target router accepts the request, it returns a keep alive message in response.

Once a neighbour relationship is established, the neighbour reachability procedure is used to maintain the relationship. Both sides needs to be assured that the other side still exists and is still engaged in the neighbour relationship. For this purpose, both routers send keep alive messages to each other.

Both sides router maintains a database of the subnetworks that it can reach and the preferred route for reaching that subnetwork. If the database exchanges, router issues an update message that is broadcast to all other routers implementing BGP. By the broadcasting of these updated message, all the BGP routers can build up and maintain routing information.

BGP connections inside an autonomous system are called internal BGP (iBGP) and BGP connections between different autonomous systems are called external BGP (eBGP).

Below figure shows the internal and external BGP.