BFD Bidirectional Forwarding Detection
A minimal heartbeat protocol that detects a dead link in milliseconds instead of seconds. Specified in RFC 5880 (2010) and deliberately dumb: it does nothing except establish alive or dead.
The problem it solves is that routing protocols' own timers are slow — OSPF notices a failure after 40 seconds by default, BGP after 90 or 180. And fibre going down between two switches gives the router no link-down signal at all. BFD sends small packets in both directions at intervals down toward 50 ms, and when three are missed it reports upward to OSPF, BGP, IS-IS, VRRP or static routes, which converge immediately. Aggressive timers require the session to be handled in hardware, otherwise a busy CPU produces false alarms.