Operation
BGP neighbors, called peers, are established by manual configuration between routers to create a TCP session on port 179. A BGP speaker will periodically (every 30 seconds) send 19-byte keep-alive messages to maintain the connection. Among routing protocols, BGP is unique in using TCP as its transport protocol.
When BGP runs between two peers in the same autonomous system (AS), it is referred to as Internal BGP (IBGP or Interior Border Gateway Protocol). When it runs between autonomous systems, it is called External BGP (EBGP or Exterior Border Gateway Protocol). Routers on the boundary of one AS exchanging information with another AS are called border or edge routers. In the Cisco operating system, IBGP routes have an administrative distance of 200 and that of EBGP is 20; IBGP is thus less preferred than either external BGP or any interior routing protocol. Other router implementations also prefer EBGP to IGPs, and IGPs to IBGP.
Read more about this topic: Border Gateway Protocol
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