S-BFD provides a mechanism to check the data path forwarding for an SR-TE LSP. If an S-BFD session fails, the LSP can be brought operationally down when the failure-action command is configured with the failover-or-down option. When the failure-action command is not configured, an S-BFD failure will only raise a trap. The failure-action command is available in the following context:
configure>router>mpls
lsp <name> sr-te
bfd
failure-action failover-or-down
no failure-action
The failure-action command is configured at the LSP level. It can be configured whether S-BFD is applied at the LSP level or the individual path level. The failure-action command can be configured even if the BFD template is not yet configured.
For LSPs configured with a primary path and a secondary or standby path and a failure action of failover-or-down, the following points apply.
The path is held in an operationally down state when its S-BFD session is down.
If all paths are operationally down, the SR-TE LSP is taken operationally down and a trap is generated.
If S-BFD is enabled at the LSP or active path level, a switchover from the active path to an available path is triggered if the S-BFD session fails on the active path (primary or standby).
If S-BFD is not enabled on the active path and this path is shut down, a switchover is triggered.
If S-BFD is enabled on the candidate standby or secondary path, this path is only selected if S-BFD is up.
An inactive standby path configured with S-BFD is only available to become active if it is not operationally down; that is, its S-BFD session is up and all other criteria for it to become operational are true. The path is held in an inactive state if the S-BFD session is down.
The system does not revert to the primary path or start a reversion timer when the primary path is either administratively down or operationally down, because the S-BFD session is not up or down for any other reason.
For LSPs configured with only one path and a failure action of failover-or-down, the following points apply.
The path is held in an operationally down state when its S-BFD session is down.
If the path is operationally down, the LSP is taken operationally down and a trap is generated.
For LSPs configured with one or more paths but no configured failure action, a BFD trap is raised when the LSP goes down. The path state does not change.