BGP uses TCP port 179 for communications. It is the only routing protocol to use TCP for its L4 protocol. (Other IGPs reside directly above the IP layer.) This allows for large messages to be sent between neighbors using the TCP sliding window capability to support the needs of the internet (rather than relying on 1:1 message acknowledgement mechanisms characteristic of IGPs). Because of TCP’s reliable transport mechanisms, the protocol has no error recovery mechanisms of its own.
BGP does not auto-discover neighbors, instead requiring neighbor relationships to be manually configured. BGP neighbors do not require direct connection. This is why they are often referred to as “BGP Peers.”
BGP supports MD5 authentication. When a neighbor relationship is first formed, there is a full exchange of routing tables. There are no periodic updates. When changes occur in the network, BGP sends triggered updates.
BGP leverages keepalive messages to maintain neighbor relationships.
Stuck-in-Active: Journal of an IT-Network Administrator 