The following describes the behavior of this feature.
When LDP resolves a primary next hop in the RTM or a remote LFA or TI-LFA backup next hop using an SR tunnel in the TTM, LDP programs a primary LDP NHLFE and a backup LDP NHLFE with an implicit null label value pointing to the SR tunnel that has the remote LFA or TI-LFA backup programmed for the same prefix.
If the LDP FEC primary next hop fails and LDP has preprogrammed a remote LFA and TI-LFA next hop with an LDP backup NHLFE pointing to an SR tunnel, the LDP ILM/LTN switches to it.
If the LDP FEC primary next hop failure impacts only the LDP tunnel primary next hop but not the SR tunnel primary next hop, the LDP backup NHLFE points to the primary next hop of the SR tunnel; the LDP ILM/LTN traffic follows this path instead of the remote LFA or TI-LFA next hop of the SR tunnel until the remote LFA or TI-LFA next hop is activated.
If the LDP FEC primary next hop becomes unresolved in the RTM, LDP switches the resolution to an SR tunnel in the TTM, if one exists, following the steps described in Stitching in the LDP-to-SR Direction.
If both the LDP primary next hop and a regular LFA next hop become resolved in RTM, the LDP FEC programs the primary NHLFE and backup NHLFE.