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Negotiation of the BFD Liveness Detection Interval

When you issue the appropriate bfd-liveness-detection command on an IS-IS, OSPF, RIP, or PIM interface, BFD liveness detection is established with all of its BFD-enabled peers. When an update is received from a peer—if BFD is enabled and if the session is not already present—the local peer attempts to create a BFD session to the remote peer.

Each pair of peers negotiates acceptable transmit and receive intervals for BFD packets. These values can be different on each peer.

The negotiated transmit interval for a peer is the interval between the BFD packets that it sends to its peers. The receive interval for a peer is the minimum time that it requires between packets sent from its peer; the receive interval is not negotiated between peers.

To determine the transmit interval, each peer compares its configured minimum transmit interval with its peer's minimum receive interval. The larger of the two numbers is accepted as the transmit interval for that peer.

Consider the following example. Router A and Router B are peers, with the following BFD liveness detection values configured.

Router	Configured Transmit Interval (ms)	Configured Receive Interval (ms)
A	400	500
B	450	450

For Router A, the negotiated transmit interval is the greater of its transmit interval (400 ms) and the Router B receive interval (450 ms), or 450 ms.
For Router B, the negotiated transmit interval is the greater of its transmit interval (450 ms) and the Router A receive interval (500 ms), or 500 ms.

The liveness detection interval is the period a peer waits for a BFD packet from its peer before declaring the BFD session to be down. The detection interval is determined independently by each peer and can be different for each. The detection interval for the local peer is calculated as the remote peer's negotiated transmit interval times the detection multiplier value configured on the remote peer.

Router	Negotiated Transmit Interval (ms)	Detection Multiplier	Liveness Detection Interval (ms)
A	450	2	1500
B	500	3	900

For Router A, the detection interval is Router B's negotiated transmit interval times the Router B detection multiplier: 500 ms x 3 = 1500 ms.
For Router B, the detection interval is Router A's negotiated transmit interval times the Router A detection multiplier: 450 ms x 2 = 900 ms.

If Router A fails to receive a BFD packet from Router B within 1500 milliseconds, Router A declares the BFD session to be down. Similarly, if Router B fails to receive a BFD packet from Router A within 900 milliseconds, Router B declares the BFD session to be down. In either case, all routes learned from the failed peer are purged immediately.

Note: Before the router can use any bfd-liveness-detection command, you must specify a BFD license key. To view an already configured license, use the show license bfd command.

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