https://www.cisco.com/c/en/us/td/docs/routers/sdwan/configuration/Monitor-And- Maintain/monitor-maintain-book/m-network.html BFD Protocol The Role of BFD in Cisco SD-WAN Solution The BFD protocol detects links failures between routers. It measures data loss and latency on the data tunnel to determine the status of the devices at either end of the connection.
For data plane resiliency, the Cisco SD-WAN software implements the BFD protocol, which runs automatically on the secure IPsec and GRE connections between routers. These connections are used for the data plane, and for data traffic, and are independent of the DTLS tunnels used by the control plane.