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# BFD Protocol and RSVP-TE

The Bidirectional Forwarding Detection (BFD) protocol uses control packets and shorter detection time limits to more rapidly detect failures in a network. Also, because they are adjustable, you can modify the BFD timers for more or less aggressive failure detection.

Without BFD, RSVP-TE can learn about adjacency failures by either of two methods. If RSVP-TE hellos are configured, then hello message timeouts indicate a failure. If hellos are not configured, then RSVP-TE learns about failures from resv and path messages.

When a BFD session exists between RSVP-TE peers, a peer that goes down is detected quickly, enabling faster rerouting of traffic. Adjacency failure detection by means of hello messages takes place on the order of seconds, whereas BFD fast failure detection can take place on the order of hundreds of milliseconds.

When you issue the mpls rsvp bfd-liveness-detection command on an RSVP-TE major interface, BFD liveness detection is established with all BFD-enabled RSVP-TE peers associated with that interface.

When an RSVP-TE session is established with the remote peer—if BFD is enabled and if the BFD session is not already present—then the local peer attempts to create a BFD session to the remote peer. The BFD session is established only if when both of the following are true:

- At least one RSVP-TE LSP exists between (passes through) a pair of directly connected RSVP-TE major interfaces.
- Both interfaces are BFD-enabled.

Consequently, when the last LSP is torn down between the interfaces, the BFD session is no longer required and is brought down as well.

Each adjacent pair of peers negotiates an acceptable transmit interval for BFD packets. The negotiated value can be different on each peer. Each peer then calculates a BFD liveness detection interval. When a peer does not receive a BFD packet within the detection interval, it declares the BFD session to be down and purges all routes learned from the remote peer.

#### Related Topics

- For general information about configuring and monitoring the BFD protocol, see JUNOSe IP Services Configuration Guide.

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