Simple definition
BFD (Bidirectional Forwarding Detection) is a lightweight protocol designed to detect loss of connectivity between two network devices very quickly.
Technical definition
Defined by RFC 5880, BFD exchanges control packets between two endpoints and maintains a session state. Transmit/receive intervals and the detection multiplier determine how quickly a session is declared down. BFD can provide this failure signal to protocols such as BGP or OSPF so they converge faster.
What is it used for?
Detect path failures faster than the native timers of many routing protocols and trigger reconvergence in hundreds of milliseconds when the infrastructure supports it.
Practical example
Two BGP routers also establish a BFD session. If the path fails and BFD packets stop arriving, BFD quickly declares the peer down; BGP can then remove routes without waiting for its normal hold timer.
Common issues
- Timers are too aggressive for the device CPU
- BFD traffic is allowed in one direction but filtered in the other
- BFD session is up but the routing protocol is not bound to it
- BFD parameters or operating mode are incompatible between peers
Key takeaway: BFD detects connectivity loss; it does not replace BGP or OSPF, but gives them a much faster failure signal.