Following are the different types of Timers present in BGP :

• Keep Alive Timer : This is the heartbeat timer, in which a local NEIGHBOUR sends a BGP heart-beat packet to a remote neighbour at regular intervals to check reachability and availability. This interval is set to '30' seconds by default.
• Hold down Timer : This is the amount of time that the local neighbour must wait before declaring the remote neighbour unavailable. This interval is set at "90" seconds by default, which is '3' TIMES the Keep-Alive Interval. In other words, if a local neighbour misses three Keep-Alive packets in a row from a remote neighbour, the local neighbour considers the remote neighbour unavailable and changes the STATUS of the neighbourship, as well as removing all associated routes advertised by the neighbour from the routing table/BGP table. Before and after the hold-down timer expires, the BGP neighbour status changes. The hold down timer is set to 90 seconds by default, and after that, the local neighbour moves through various stages like 'idle,' 'connect,' and 'active.' The status changes to 'Idle' at first, then to 'Connect' after 5 seconds, and then to 'Active' after 10 seconds.
• Advertisement Interval : The BGP Advertisement Interval is a timer that determines how much time must pass between a route being advertised and being removed from a BGP peer. For eBGP peers, the default is 30 seconds, and for iBGP peers, it's 5 seconds. This can be modified on a per-neighbor basis.