This feature allows an SR tunnel to be used as a remote LFA or TI-LFA backup tunnel next hop by an LDP FEC. The feature is enabled using the CLI command string config>router>ldp>fast-reroute backup-sr-tunnel. See LDP Commands for more information.
This feature requires the LDP-to-Segment Routing Stitching for IPv4 /32 Prefixes (IS-IS) feature as a prerequisite, because the LSR performs the stitching of the LDP ILM to an SR tunnel when the primary LDP next hop of the FEC fails. Therefore, LDP monitors SR tunnels programmed by the IGP in the TTM without the need for a mapping server.
It is assumed that:
the backup-sr-tunnel option is enabled in LDP
the loopfree-alternates ti-lfa or loopfree-alternates remote-lfa option is enabled in the IGP instance (see the 7705 SAR Routing Protocols Guide, ‟IS-IS Command Reference”)
The loopfree-alternates options can be enabled separately or together. If both options are enabled, TI-LFA backup takes precedence over remote LFA backup.
LDP was able to resolve the primary next hop of the LDP FEC in the RTM
If the IGP LFA SPF does not find a regular LFA backup next hop for an LDP FEC prefix, it runs the TI-LFA and remote LFA algorithms. If the IGP LFA SPF finds a remote LFA or TI-LFA tunnel next hop, LDP programs the primary next hop of the FEC using an LDP NHLFE and programs the remote LFA or TI-LFA backup tunnel next hop using an LDP NHLFE pointing to the SR tunnel endpoint.
The LDP packet is not sent over the SR tunnel. The LDP label is stitched to the segment routing label stack. LDP points both the LDP ILM and the LTN to the backup LDP NHLFE, which uses the SR tunnel endpoint.