What is Bidirectional Forwarding Detection (BFD) on Junos ?

The Bidirectional Forwarding Detection (BFD) protocol is a simple mechanism that detects failures in a network and works in a wide variety of network environments and topologies. In BFD operation, switches exchanges BFD hello packets at a specified interval and detect a neighbor failure if they do not receive a reply after a specified interval. The BFD failure detection timers support shorter time limits than the static route failure detection mechanisms, so they can provide faster detection of failures. 

To configure faster failure detection, use lower BFD timer values. The timers can automatically adapt to a higher value if an adjacency fails, and they also adapt to a higher value when a BFD session flap occurs more than three times in a span of 15 seconds.

In this case, a back-off algorithm increases the receive interval by two if the local BFD instance is the reason for the session flap and increases the transmission interval by two if the remote BFD instance is the reason for the session flap. 

You can use the clear bfd adaptation command to return BFD interval timers to their configured values. This command is hitless, meaning that it does not affect traffic flow.