The Topology-Independent LFA (TI-LFA) feature improves the protection coverage of a network topology by computing and automatically instantiating a repair tunnel to a Q node which is not in the shortest path from the computing node. The repair tunnel uses the shortest path to the P node and a source-routed path from the P node to the Q node.
In addition, the TI-LFA algorithm selects the backup path that matches the post-convergence path. This helps capacity planning in the network because traffic always flows on the same path when transitioning to the FRR next hop and then on to the new primary next hop.
At a high level, the TI-LFA link protection algorithm searches for the closest Q node to the computing node and then selects the closest P node to this Q node, up to a maximum number of labels. This is performed on each of the post-convergence paths to each destination node or prefix D.
When the TI-LFA feature is enabled in IS-IS, it provides a TI-LFA link-protect backup path in IS-IS MT=0 for an SR-ISIS IPV4/IPv6 tunnel (node SID and adjacency SID), for an IPv4 SR-TE LSP, and for LDP IPv4 FEC when the LDP fast-reroute backup-sr-tunnel option is enabled.