BGP sends 19 byte long KEEPALIVE messages at an interval specified by the KEEPALIVE Interval timer in the BGP router configuration (the default is 60 seconds). If a BGP peer misses three keepalives (180 seconds) all routes from that peer are suppressed according to the HOLD DOWN timer setting.
The HOLD DOWN timer determines how long a peer should wait to hear a KEEPALIVE or UPDATE message before assuming the neighbor is down, shutting down the BGP session, and withdrawing all routes from it's announcements.
If the negotiated Hold Down timer value is zero, no KEEPALIVES will be sent. This may prevent a BGP session from becoming established depending upon the manufacturer's implementation, or it may cause packets to be dropped as they are forwarded to a BGP device that is no longer operational, but which could not be detected because there are no KEEPALIVE messages being exchanged.