Once a BGP session has been established, the BGP peers exchange route advertisements by means of update messages. Update messages contain a one or more route advertisements, and they can contain one or more prefixes that are to be removed from the BGP routing table. If the peers need to advertise multiple routes, they generate and send multiple update messages as they detect changes to the network. In the absence of changes to the routing table, no update messages are generated.
While a BGP session is active, each router on the BGP session generates keepalive messages periodically. The timing of these messages is determined by the hold time on the session. The hold time is a negotiated value specifying the number of seconds that can elapse without keepalive messages before BGP designates the link inactive. Three messages are sent during every hold time interval.
When a peer connection is closed (either by error or if the BGP session is closed), a notification message is generated and sent to the peer router that did not experience the error or did not terminate the BGP session.