Which forwarding mechanism is used to monitor tunnel health and enable rapid failover?

• A. Bidirectional Forwarding Detection (BFD)
• B. Spanning Tree Protocol (STP)
• C. LLDP
• D. OSPF

Answer: (A)

Fast, reliable tunnel health detection for rapid failover in SD-WAN relies on a lightweight liveness mechanism that checks connectivity between tunnel endpoints. Bidirectional Forwarding Detection provides exactly that: it sends small, frequent keepalive messages between the two ends of a tunnel and considers the tunnel healthy as long as responses arrive within a configured interval. If these messages stop, the tunnel is declared failed and traffic can switch almost immediately to a backup path, minimizing disruption. Because BFD runs independently of routing protocols and introduces minimal overhead, detection times are in the low milliseconds, enabling near-instant failover. Spanning Tree Protocol is a Layer 2 loop-prevention mechanism with slower convergence and is not designed for rapid WAN tunnel health monitoring. LLDP is for device discovery and topology information, not for monitoring tunnel health. OSPF is a routing protocol whose convergence is slower than dedicated fast-failover mechanisms like BFD, making it less suitable for immediate tunnel failover.