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9.2 | ![]() |
Basic BGP Operation | |
9.2.2 | ![]() |
BGP neighbors |
When two routers establish a TCP enabled BGP connection, they are
called neighbors or peers. Each router running BGP is called a BGP
speaker. Peer routers exchange multiple messages to open and confirm
the connection parameters, such as the version of BGP to be used. If
there are any disagreements between the peers, notification errors are
sent and the connection fails.
When BGP neighbors first establish a connection,
Peers advertise destinations that are reachable through them by using update messages. These messages contain route prefix, AS path, path attributes such as the degree of preference for a particular route, and other properties. The information for network reachability can change, such as when a
route becomes unreachable or a better path becomes available. BGP informs its
neighbors of this by withdrawing the invalid routes and injecting the new
routing information.
If there are no routing changes to transmit to a peer, a BGP speaker will periodically send keepalive messages to maintain the connection. These 19-byte keepalive packets are sent every 60 seconds by default. These packets present a negligible drain on bandwidth and the CPU time on a router.
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