How is a sub-second failure of a transport link detected in a Cisco SD-WAN network?

How is a sub-second failure of a transport link detected in a Cisco SD-WAN network?

A. Hellos are sent between the WAN Edge routers and the vSmart controller. B. BFD runs on the IPsec tunnels between WAN Edge routers. C. BGP is used between WAN Edge routers and the vSmart controller. D. Link state change messages are sent between vSmart controllers.