Which two modes of operation does BFD support?

A session may operate in one of two modes: asynchronous mode and demand mode. In
asynchronous mode, both endpoints periodically send Hello packets to each other. If a number of
those packets are not received, the session is considered down.
In demand mode, no Hello packets are exchanged after the session is established; it is assumed
that the endpoints have another way to verify connectivity to each other, perhaps on the
underlying physical layer. However, either host may still send Hello packets if needed.
Reference: http://en.wikipedia.org/wiki/Bidirectional_Forwarding_Detection