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ISIS Fast Reroute Convergence

Sub-second service restoration is a key requirement for MPLS and native IP-based network service providers. There are many ways to achieve fast reroute with sub optimal next-hop to reach destination like loop-free alternate and remote loop-free alternate. In these cases, IGP will download primary and backup next-hop beforehand in FIB. Packet forwarding engine (PFE) performs local repair when a primary next-hop loses its reachability to a given destination. Since PFE already has alternative path to reach destination, sub-second restoration is possible. If destination is reachable via equal cost multi path (ECMP) then only primary path is downloaded to forwarding information base (FIB). If few ECMP links go down than the required bandwidth for a destination, fast reroute convergence is not possible.

To resolve this, the best ECMP links are grouped as unilist of primary next-hops to reach destination and the sub-optimal ECMP links are grouped as unilist of backup next-hops to reach the destination. If bandwidth of the primary next-hops falls below the desired bandwidth, PFE does a local repair and switches traffic to backup unilist next-hops. This is yet another backup, where the backup path is computed and installed in FIB for ECMP paths. Here, a set of best ECMP links are grouped as primary next-hops to reach destination and a set of sub-optimal ECMP links are grouped as backup next-hops to reach destination. If bandwidth of the primary next-hops falls below desired bandwidth due to link failure on primary group, PFE should perform local repair and switch the traffic to backup next-hops.

In the following topology, R1 has three ECMP links to D1 via R2. R1 also has three sub-optimal ECMP links to D1 via R3 and R2. All ECMP links L1, L2 and L3 can be placed under one group; a primary group and also group sub-optimal ECMP links L3, L4 and L5 under another group; a backup group.

Figure 1: TopologyTopology

IS-IS calculates the shortest path using shortest-path-first (SPF) algorithm and downloads primary next-hops with appropriate weight in FIB. IS-IS also calculates backup next-hops and downloads them to FIB with appropriate weight.

Backup next-hops weight will always be greater than primary next-hops. If a link from primary group goes down, PFE performs a local repair and modifies the weight of the next-hops. PFE forwards traffic to the destination with least weight next-hops to achieve sub-millisecond convergence. IS-IS runs SPF and comes up with a set of primary and backup next-hops. IS-IS then updates the FIB with the updated next hops. PFE resumes traffic forwarding on new next-hops without any traffic loss.