Neighbor Relationships
Most advanced routing protocols have some system of neighbor discovery, generally a hello protocol, so that a router can discover neighbors and trade routing information reliably. BGP is an exception because it requires the manual configuration of neighbor relationships; it does not discover neighbors automatically.
Like other advanced routing protocols, though, BGP requires a reliable transport system to guarantee that packets don't get lost between peers. BGP uses TCP for reliable transport.
When a router running BGP (a BGP speaker) is configured to build a neighbor relationship with another BGP speaker, it first builds a TCP connection to transport information. (Port 179 is the well-known port for BGP.) This means that ...
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