LSP Active Path Failover Triggers

The active path of an LSP is switched to an alternative path in the following cases:

the active path goes into degraded state because of FRR or soft preemption
the active path is degraded because the BFD session is going from up to down (only applicable if the failure action is set to failover or failover-or-down)
reverting from a secondary or standby path to the primary path (with or without a reverter time configured)
switching between secondary or standby paths because of path preference
switching between secondary or standby paths because of the tools perform router mpls switch-path or force-switch-path commands
switching because of an MBB on the active path where the old and new path have the same bfd-enable configuration
switching from the primary path to secondary or standby paths because of the tools perform router mpls manual-switch-path command

Table 1 describes path switchover events depending on the failure action configuration.

Table 1. Path Switchover Triggers based on BFD Failure Action Configuration
BFD failure-action configuration	Old active path		New active path	Switchover to new path
BFD failure-action configuration	bfd-enable configuration at LSP or path	BFD session state	bfd-enable configuration at LSP or path	Switchover to new path
no failure-action failure-action failover	Any	Any	Any	Switch immediately without checking the BFD session state on new path.
failure-action down	BFD enabled	BFD session up	BFD enabled	Wait for a maximum of 10 seconds for the BFD session to come up on the new path before switching. If the BFD session does not come up on the new path after 10 seconds, switch anyway.
		BFD session up	BFD disabled	Switch immediately without checking the BFD session state on new path.
		BFD session down	BFD enabled	Switch immediately without checking the BFD session state on new path.
		BFD session down	BFD disabled	Switch immediately without checking the BFD session state on new path.
	BFD disabled	—	BFD enabled	Wait for a maximum of 10 seconds for the BFD session to come up on the new path before switching. If the BFD session does not come up on the new path after 10 seconds, switch anyway.
	BFD disabled	—	BFD disabled	Switch immediately without checking the BFD session state on new path.

For failure-action failover-or-down, a path is in the degraded state if it has BFD enabled and the BFD session is not up. Switching between primary, standby, and secondary paths of the LSP follows rules of the best path selection algorithm, for example, a non-degraded path is better than a degraded path and a degraded primary is better than a degraded standby or secondary path. Because the BFD degraded state affects LSP active path selection, waiting for BFD to come up on new path is already accounted for and these cases have been excluded from Table 2.

Switching to an MBB path requires waiting for the BFD session to come up on the new MBB path. These cases are described in Table 2. This applies to MBB on both active and inactive paths to reduce the toggling of a BFD degraded state on the path.

Table 2. MBB Path Switching with failure-action failover-or-down
BFD failure-action configuration	Old path		New MBB path	Switching to new path
BFD failure-action configuration	bfd-enable configuration at LSP or path	BFD session state	bfd-enable configuration at LSP or path	Switching to new path
failure-action failover-or-down	BFD enabled	BFD session up	BFD enabled	Wait for a maximum of ‟`w`” seconds for the BFD session to come up on the new path before switching. If the BFD session does not come up on the new path after ‟`w`” seconds, switch anyway. Where `w` is the BFD wait-for-up-timer from the context where BFD is enabled.
	BFD enabled	BFD session up	BFD disabled	This case is not applicable because the MBB path has same BFD configuration as existing path.
	BFD enabled	BFD session down	BFD enabled	Switch immediately without checking the BFD session state on new path.
	BFD enabled	BFD session down	BFD disabled	This case is not applicable because the MBB path has same BFD configuration as existing path.
	BFD disabled	—	BFD enabled	This case is not applicable because the MBB path has the same BFD configuration as existing path.
	BFD disabled	—	BFD disabled	Switch immediately without checking the BFD session state on new path.