Info

BGP is a path vector protocol.
Uses TCP

Routing updates carry a list of autonomus system numbers and aggregated prefix addresses as well as some policy routing information.

BGP4 is conntection-orientated, neighbors are establised and maintained using BGP-4 probe keepalives. These keepalives are the 19 byte header used in the BGP updates.

BGP uses four message types:
Open messages, Keepalives, updates, notifications

There are two types of BGP4:
Internal BGP4 (IBGP4)
External BGP4 (EBGP4)

BGP4 Syncronization

Before IBGP4 can propogate a route into another AS handing it over to EGBP4, the route must be totally nokwn within the AS.  The Internal Gateway Protocol (IBGP) or the internal routing protocol must be synchronized with BGP4.