Which protocol is used in Cisco SD-WAN to measure link quality?

BFD (Bidirectional Forwarding Detection) is the correct answer because it's a lightweight, low-overhead protocol specifically designed to detect failures in the forwarding path between two routing devices. In Cisco SD-WAN, BFD sessions run over the IPsec tunnels to continuously monitor the health and performance of the underlying transport links, providing rapid fault detection crucial for dynamic path selection and high availability. IPsec is incorrect because it provides secure communication (encryption, authentication) but doesn't inherently measure link quality or detect failures. OMP (Overlay Management Protocol) is incorrect because it's the routing protocol for the SD-WAN overlay, exchanging reachability information and policies, not directly measuring link quality. RSVP (Resource Reservation Protocol) is incorrect because it's used for reserving network resources for QoS, not for link failure detection.