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Configuring BFD for PIM

The Bidirectional Forwarding Detection (BFD) Protocol is a simple hello mechanism that detects failures in a network. BFD works with a wide variety of network environments and topologies. A pair of routing devices exchanges BFD packets. Hello packets are sent at a specified, regular interval. A neighbor failure is detected when the routing device stops receiving a reply after a specified interval. The BFD failure detection timers have shorter time limits than the Protocol Independent Multicast (PIM) hello hold time, so they provide faster detection.

The BFD failure detection timers are adaptive and can be adjusted to be faster or slower. The lower the BFD failure detection timer value, the faster the failure detection and vice versa. For example, the timers can adapt to a higher value if the adjacency fails (that is, the timer detects failures more slowly). Or a neighbor can negotiate a higher value for a timer than the configured value. The timers adapt to a higher value when a BFD session flap occurs more than three times in a span of 15 seconds. A back-off algorithm increases the receive (Rx) interval by two if the local BFD instance is the reason for the session flap. The transmission (Tx) interval is increased 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. The clear bfd adaptation command is hitless, meaning that the command does not affect traffic flow on the routing device.

You must specify the minimum transmit and minimum receive intervals to enable BFD on PIM.

To enable failure detection:

  1. Configure the interface globally or in a routing instance.

    This example shows the global configuration.

  2. Configure the minimum transmit interval.

    This is the minimum interval after which the routing device transmits hello packets to a neighbor with which it has established a BFD session. Specifying an interval smaller than 300 ms can cause undesired BFD flapping.

  3. Configure the minimum interval after which the routing device expects to receive a reply from a neighbor with which it has established a BFD session.

    Specifying an interval smaller than 300 ms can cause undesired BFD flapping.

  4. (Optional) Configure other BFD settings.

    As an alternative to setting the receive and transmit intervals separately, configure one interval for both.

  5. Configure the threshold for the adaptation of the BFD session detection time.

    When the detection time adapts to a value equal to or greater than the threshold, a single trap and a single system log message are sent.

  6. Configure the number of hello packets not received by a neighbor that causes the originating interface to be declared down.
  7. Configure the BFD version.
  8. Specify that BFD sessions should not adapt to changing network conditions.

    We recommend that you not disable BFD adaptation unless it is preferable not to have BFD adaptation enabled in your network.

  9. Verify the configuration by checking the output of the show bfd session command.

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