Configuration of SR Policy S-BFD and Mode Parameters

S-BFD and protection mode parameters are configured in a named maintenance policy. This is applied to SR policy paths that are imported by BGP as a policy statement action or by binding to a static SR policy configuration.

Maintenance policies are configured as follows:

configure>router>segment-routing>
   maintenance-policy <name> 
      bfd-enable  
      bfd-template <name>
      mode {linear | ecmp-protected} 
      threshold <number>    
      revert-timer <timer-value>  
      hold-down-timer <timer-value> 
      no shutdown

The bfd-enable command enables or disables BFD on all of the segment lists of the candidate path that are installed in the data path.

The bfd-template command refers to a named BFD template that must exist on the system.

The mode command specifies how to program the data path and how to behave if the number of BFD sessions that are up is less than the threshold and the hold-down timer has expired. All of the paths in the set must have the same mode (see SR Policy Route and Candidate Path Parameter Consistency). All of the allowed segment lists of the SR policy path are programmed in the IOM. The default mode is none.

In both the linear mode and ecmp-protected modes, if two or more SR policy paths with the same {headend, color, endpoint} have the same mode, then the highest preference path is treated as an effective primary path while the next highest path preference is treated as the standby path. If a third path is present in the linear mode, then this is treated as a tertiary path and also programmed in the IOM.

In the ecmp-protected mode, all the segment lists of the top two best preference paths are programmed it the IOM. However, in linear mode, the lowest index segment list of each of the top three preference paths is programmed in the IOM and linear protection is supported between that set. All of the segment lists of the programmed paths are made available for forwarding S-BFD packets.

If the currently active path becomes unavailable due to S-BFD, the system fails over to the next best preference candidate path that is available. If all programmed candidate paths are unavailable, then the SR policy is marked as down in TTM.

The linear mode supports uniform failover between candidate paths (policy routes) of the same SR policy. If linear mode is configured then the following rules apply.

The threshold command configures the minimum number of S-BFD sessions that must be up to consider the SR policy candidate path to be up. If it is below this number, the SR policy candidate path is marked as BFD degraded by the system. The threshold parameter is only valid in the ecmp-protected mode (a threshold of 1 is implicit in the linear mode).

If the revert-timer command is configured, then the router starts a revert timer when the primary path recovers (for example, after the number of S-BFD sessions that are up is ≥ threshold and the hold-down timer has expired) and switches back when the timer expires. If no revert-timer is configured, then the system reverts to the primary path for the policy when it is restored.

If a secondary or tertiary path is currently active, and the revert timer is started (due to recovery of the primary path), but the secondary path subsequently goes down due to the number of up S-BFD sessions being less than the threshold, and no other better preference standby path is available, then the router reverts immediately to the primary path. However, if a better preference standby path is available and up, the revert timer is not canceled and the system switches to the better preference standby path and switches back to the primary path when the revert timer expires. If the hold-down timer is currently active on a better-preference path, then the system immediately switches to the primary path. If the system needs to switch to the primary path but the hold-down timer is still active on the primary path, the system cancels the timer and switches immediately.

The hold-down-timer command is intended to prevent bouncing of the SR policy path state if one or more BFD sessions associated with segment lists flap and cause the threshold to be repeatedly crossed in a short period of time. The hold-down timer is started when the number of S-BFD sessions that are up drops below the threshold. The SR policy path is not considered to be up again until the hold-down timer has expired and the number of S-BFD sessions that are up equals or exceeds the threshold.

A maintenance policy can only be deleted or a value changed if the maintenance policy is administratively disabled (shutdown). A maintenance policy can only be enabled if the bfd-enable, bfd-template and mode commands are configured. All associated SR policy paths are deleted from the IOM if a maintenance template is shutdown.

Parent topic: Configuration of S-BFD and Protection for SR Policies