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 since traffic always flows on the same path when transitioning to the FRR next-hop and then onto 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 a SR-ISIS IPV4/IPv6 tunnel (node SID and adjacency SID), for a IPv4 SR-TE LSP, and for LDP IPv4 FEC when the LDP fast-reroute backup-sr-tunnel option is enabled.