Which failure-detection mechanism does BFD use?

Bidirectional Forwarding Detection (BFD) operates by sending BFD control packets at a consistent, fixed interval between two endpoints. This consistent rate of packet exchange allows for rapid and predictable detection of forwarding path failures. If a configured number of consecutive BFD packets are not received within the expected interval, the session is declared down, and the routing protocol (e.g., OSPF, EIGRP, BGP) is notified to converge quickly. "Layer 2 protocol failure" and "routing protocol failure" are consequences that BFD helps to detect or mitigate, not the mechanism BFD itself uses for detection. While BFD can operate in a "variable rate" mode (asynchronous mode), its primary and most effective failure-detection mechanism, especially for rapid detection, relies on a consistent, predefined interval for sending and expecting packets.