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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, they exchange all candidate BGP routes. After this initial exchange, incremental updates are sent as network information changes. As discussed in earlier modules, incremental updates are more efficient than complete table updates. This is especially true with BGP routers, which may contain the complete Internet routing table.

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. Withdrawn routes are part of the update message. BGP routers keep a table version number that tracks the version of the BGP routing table received from each peer. If the table changes, BGP increments the table version number. A rapidly incrementing table version is usually an indication of instabilities in the network, or a misconfiguration.

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.