Configuring Topology-Independent Loop-Free Alternate with Segment Routing for IS-IS

Loop-free alternate (LFA) and remote LFA have been used to provide fast-reroute protection for several years. With LFA, a point of local repair (PLR) determines whether or not a packet sent to one of its direct neighbors will reach its destination without looping back through the PLR. In a typical network topology, perhaps 40-60 percent of destinations can be protected by LFA. Remote LFA expands on the concept of LFA by allowing the PLR to impose a single label to tunnel the packet to a repair tunnel endpoint from which the packet can reach its destination without looping back through the PLR. Using remote LFA, more destinations can be protected by the PLR compared to LFA. However, depending on the network topology, the percentage of destinations protected by remote LFA usually less than 100 percent.

Topology-independent loop-free alternate (TI-LFA) extends the concept of LFA and remote LFA by allowing the PLR to use deeper label stacks to construct backup paths. In addition, TI-LFA imposes the constraint that the backup path used by the PLR be the same path that a packet takes once the IGP converges for a given failure scenario. This path is referred to as the post-convergence path.

Using the post-convergence path as the backup path has some desirable characteristics. For some topologies, a network operator only needs to make sure that the network has enough capacity to carry the traffic along the post-convergence path after a failure. In these cases, a network operator does not need to allocate additional capacity to deal with the traffic pattern immediately after the failure while the backup path is active, because the backup path follows the post-convergence path.