A path is first configured with S-BFD. This path is held operationally down and not added to the TTM until BFD comes up (subject to the BFD wait time).
The BFD wait-for-up-timer command provides a mechanism that cleans up the LSP path state at the head end in both cases where S-BFD does not come up in the first place and where S-BFD goes from up to down. This timer is started when BFD is first enabled on a path or an existing S-BFD session transitions from up to down. When this timer expires and if S-BFD is not up, the path is torn down by removing it from the TTM and the IOM and the LSP retry timer is started.
In the case where S-BFD goes from up to down, if there is only one path, the LSP is removed immediately from the TTM when S-BFD fails, then is deprogrammed when the wait-for-up-timer expires.
If all the paths of an LSP are operationally down because of S-BFD, then the LSP is taken operationally down and removed from the TTM and the BFD wait-for-up-timer is started for each path. If one or more paths do not have S-BFD configured on them, or are otherwise not down, then the LSP is not taken operationally down.
When an existing S-BFD session fails on a path and the failure action is failover-or-down, the path is put into the operationally down state. This state and reason code are displayed in a show>router>bfd>seamless-bfd command and a trap is generated. The configured failure action is then enacted.