Loop-free convergence using ordered FIB updates: Analysis and routing optimization

Intradomain IP routing protocols like OSPF or IS-IS are robust against failures. New fast reroute mechanisms can minimize packet loss directly after an outage by temporarily detouring packets around the failed network element. This buys time for a loop-free convergence, which brings the topology to a new stable path layout without causing temporary micro-loops. We analyze the link utilizations during this loop-free convergence phase using the ordered FIB updates (OFIB) mechanism. We show that depending on the router update order, OFIB can temporarily increase the utilization on certain links in the network. To minimize the temporary load increase, we present a heuristic link cost optimization that minimizes the link utilizations both during failure-free routing and all phases of a failure recovery process. As OFIB does not define a unique global update order but provides only local constraints, it is difficult to calculate the highest possible link utilizations. We introduce a tight upper bound to the maximum link utilization, independent of the actual update order. It can quickly be calculated and allows us to perform link cost optimization including the loop-free convergence phase. We show that this results in a routing configuration that avoids additional overload during the OFIB phase without impairing normal routing performance.