CCNP Certification / BSCI Exam Tutorial: The BGP Neighbor Process
Like TCP, BGP is connection-oriented. An underlying connection between two BGP speakers is established before any routing information is exchanged. This connection takes place on TCP port 179. As with EIGRP and OSPF, keepalive messages are sent out by the BGP speakers in order to keep this relationship alive.
Once the connection is established, the BGP speakers exchange routes and synchronize their tables. After this initial exchange, a BGP speaker will only send further updates upon a change in the network topology.
The IGP protocols that use Autonomous Systems, IGRP and EIGRP, require prospective neighbors to be in the same AS. This is not true with BGP. Routers can be in different Autonomous Systems and still exchange routes. The BGP neighbors do not have to be directly connected, and often are not, but do need to be able to reach the IP addresses they use in their neighbor statements.
A BGP peer that is in the same AS is referred to as an Internal BGP (iBGP) Peer, where a BGP peer in another AS is an External BGP (eBGP) Peer.
A sample iBGP...
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