正解:B
BFD provides a low-overhead, short-duration method of detecting failures in the forwarding path
between two adjacent routers, including the interfaces, data links, and forwarding planes. BFD is
a detection protocol that you enable at the interface and routing protocol levels. Cisco supports
the BFD asynchronous mode, which depends on the sending of BFD control packets between
two systems to activate and maintain BFD neighbor sessions between routers. Therefore, in
order for a BFD session to be created, you must configure BFD on both systems (or BFD peers).
Once BFD has been enabled on the interfaces and at the router level for the appropriate routing
protocols, a BFD session is created, BFD timers are negotiated, and the BFD peers will begin to
send BFD control packets to each other at the negotiated interval.