The main mechanism BGP uses to make sure neighbors are still alive is using the hold time and KEEPALIVE messages. The hold time specifies how long a router will wait for incoming BGP messages before it assumes the neighbor is dead. When a router has no UPDATE messages to send, it periodically sends KEEPALIVE messages, which, well, keep the BGP session alive.
The default value for the hold time suggested in the BGP specification (RFC 4271) is 90 seconds, and keepalives should be sent at intervals of one third the hold time (30 seconds). However, Cisco uses defaults of 180 and 60 seconds. So when two Cisco routers have established a BGP session and exchanged prefixes, 60 seconds later they’ll each send a KEEPALIVE message. Upon reception of the keepalive by the other router, that router’s hold time for the session will have counted down from 180 to 120, but it now gets reinitialized to 180. This continues every 60 seconds. However, should router A lose power, then router B won’t see any keepalives. So after 180 seconds, router B decides that router A is dead, sends a NOTIFICATION message and tears down the session.
In this case we do not know which one having issue, you need to do step by step diagnosis. between device right from connectivity.
