Users can enable TI-LFA in an IS-IS instance using one of the following command:
config>router>isis>loopfree-alternates [remote-lfa [max-pq-cost value]] [ti-lfa [max-sr-frr-labels value]]
When the ti-lfa option is enabled in IS-IS, it provides a TI-LFA link-protect backup path in IS-IS MT=0 for an SR-ISIS IPV4 and IPv6 tunnel (node SID and adjacency SID), for a IPv4 SR-TE LSP, and for an LDP IPv4 FEC when the LDP fast-reroute backup-sr-tunnel option is enabled. See more details of the applicability of the various LFA options in LFA Protection Option Applicability.
The max-sr-frr-labels parameter limits the search for the LFA backup next-hop:
0 — The IGP LFA SPF restricts the search to TI-LFA backup next-hop which does not require a repair tunnel, meaning that P node and Q node are the same and match a neighbor. This is also the case when both P and Q node match the advertising router for a prefix.
1 to 3 — The IGP LFA SPF widens the search to include a repair tunnel to a P node which itself is connected to the Q nodes with a 0-to-2 hops for a total of maximum of three labels: one node SID to P node and two adjacency SIDs from P node to the Q node. If the P node is a neighbor of the computing node, its node SID is compressed and meaning that up to three adjacency SIDs can separate the P and Q nodes.
2 (default) — Corresponds to a repair tunnel to a non-adjacent P which is adjacent to the Q node. If the P node is a neighbor of the computing node, then the node SID of the P node is compressed and the default value of two labels corresponds to two adjacency SIDs between the P and Q nodes.
When the user attempts to change the max-sr-frr-labels parameter to a value that results in a change to the computed FRR overhead, then IGP checks that all SR-TE LSPs can properly account for the overhead based on the configuration of the LSP max-sr-labels and additional-frr-labels parameter values or the change is rejected.
The FRR overhead is computed by IGP and its value is set as follows:
0 if segment-routing is disabled in the IGP instance
0 if segment-routing is enabled but remote-lfa is disabled and ti-lfa is disabled
1 if segment routing is enabled and remote-lfa is enabled but ti-lfa is disabled, or if segment-routing is enabled and remote-lfa is enabled and ti-lfa is enabled but ti-lfa max-sr-frr-labels labels is set to 0.
The value of ti-lfa max-sr-frr-labels labels, if segment-routing is enabled and ti-lfa is enabled regardless if remote-lfa is enabled or disabled.