IS-IS FRR route convergence

IS-IS fast reroute (FRR) route convergence enables you to restore sub-second service. Sub-second service restoration is a key requirement for service providers on MPLS and native IP-based networks. There are many ways to achieve fast reroute with suboptimal next hop to reach a destination, such as loop-free alternate (LFA) and remote loop-free alternate (RLFA). In these cases, IGP downloads the primary and backup next hops beforehand in the forwarding information base (FIB). The Packet Forwarding Engine does a local repair when the primary next hop loses its reachability to a given destination. Because the Packet Forwarding Engine already has an alternative path to reach its destination, sub-second restoration is possible. If the destination is reachable through equal-cost multipath (ECMP), only the primary path is downloaded to the FIB. When the bandwidth of the ECMP links is lower than the required bandwidth for a destination, fast convergence is not possible. The best ECMP links are grouped as a unilist of primary next hops to reach the destination. Suboptimal ECMP links are grouped as a unilist of backup next hops to reach the destination. If the bandwidth of the primary next hops falls below the desired bandwidth, the Packet Forwarding Engine does a local repair and traffic switch to back up the unilist next hops.