The Effect of Combining Loop-Free Alternates and Not-Via Addresses in IP Fast Reroute

The IETF currently discusses fast reroute mechanisms for IP networks (IP FRR) to accelerate the recovery in case of network element failures and to avoid microloops during network-wide routing re-convergence. Several mechanisms are proposed. Loop-free alternates (LFAs) are simple, but they cannot cover all single link and node failures. Not-via addresses are more complex and cover all single failures, but they potentially lead to longer backup paths and require tunnelling which may reduce the forwarding speed of the routers. In addition, they increase the size of the forwarding tables. This work studies the combination of those simple and complex mechanisms to achieve full single failure coverage with least overhead. First, we establish a taxonomy for LFAs according to their ability and propose combination options with not-vias for different resilience requirements. Then, we quantify the effect of combining both mechanisms regarding their applicability for the resilience requirements, routing table size, link utilization, backup path length, and amount of traffic requiring decapsulation per router. The results show that there are no strong advantages of the combined application of both mechanisms over not-via as the only IP FRR concept if 100% coverage for single link and node failures is required.