MPLS Guide Label Distribution Protocol
3HE 18686 AAAB TQZZA © 2022 Nokia.
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5.1.14.3 TTL Propagation and ICMP Tunneling
When stitching is performed between an LDP FEC and an SR IS-IS node SID tunnel,
the TTL of the outer LDP or SR label is decreased, similar to a regular swapping
operation at an LSR.
5.1.15 LDP FRR Remote LFA and TI-LFA Backup Using an
SR Tunnel for IPv4 /32 Prefixes (IS-IS)
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 (refer to the 7705 SAR Routing Protocols Guide,
“IS-IS Command Reference”)
• 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.
Note: The loopfree-alternates options can be enabled separately or together. If both
options are enabled, TI-LFA backup takes precedence over remote LFA backup.
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