Mike Shand

Disruption Free Topology Reconfiguration in OSPF Networks

A few modifications to software and/or hardware of routers have been proposed recently to avoid the transient micro loops that can occur during the convergence of link-state interior gateway protocols like IS-IS and OSPF. We1 propose in this paper a technique that does not require modifications to ISIS and OSPF, and that can be applied now by ISPs. Roughly, in the case of a manual modification of the state of a link, we progressively change the metric associated with this link to reach the required modification by ensuring that each step of the progression will be loop-free. The number of changes that are applied to a link to reach the targeted state by ensuring the transient consistency of the forwarding inside the network is minimized. Analysis performed on real regional and tier-1 ISP topologies show that the number of required transient changes is small. The solution can be applied in the case of link metric updates, manual set up, and shut down of links.