Scheduling routing table calculations to achieve fast convergence in OSPF protocol

Fast convergence to topology changes is now a key requirement in routing infrastructures while reducing the routing protocol’s processing overhead continues to be as important as before. In this paper, we examine the problem of scheduling routing table updates in link state routing protocols. Commercial routers typically use a hold time based scheme to limit the number of routing table updates as new LSAs arrive at the router. The hold time schemes limit the number of routing table updates at the expense of increased delay in convergence to the new topology, which is clearly not acceptable any more. We analyze the performance of different hold time schemes and propose a new approach to schedule routing table updates, called LSA Correlation. Rather than using individual LSAs as triggers for routing table updates, LSA Correlation scheme correlates the information in the LSAs to identify the topology change that led to their generation. A routing table update is performed when a topology change has been identified. The analysis and simulation results presented in this paper suggest that the LSA Correlation scheme performs much better than the hold time based schemes for both isolated and large scale topology change scenarios.