BGP4 Peering
A device configured to run BGP4 is called a BGP
"speaker." A BGP speaker exchanges routing information with another speaker (in the same
or a different autonomous system) by using a TCP connection to port 179 (the well-known
BGP port). The TCP connection is maintained throughout the peering session. While the
connection between BGP peers is alive, two peers communicate by means of the following
types of messages:
- OPEN
- UPDATE
- KEEPALIVE
- NOTIFICATION
- ROUTE REFRESH
A BGP4 session between peers in the same
autonomous system is an Interior BGP (iBGP) session. A session between peers in
different autonomous systems is an Exterior BGP (eBGP) session.
To establish a TCP connection between two iBGP
peers, IP reachability should be established by means of the underlying IGP protocol
(for example, OSPF) or by means of static routes. When routes are advertised in iBGP
peers, the following actions do not usually occur:
- Routes learned from an iBGP peer
are not usually advertised to other iBGP peers, to prevent loops in an
autonomous system.
- Path attributes are not usually
changed, to maintain the best path selection at other nodes in an autonomous
system.
- The autonomous system path and
next hop are not usually changed.
