Faster link-state IGP convergence and improved network scalability and stability

Abstract

It is desirable to have fast detection of failures followed by fast restoration and recovery in data networks (IP, ATM, frame relay, etc.) using link-state interior gateway protocols (IGP). This requires faster hello exchanges (unless one relies on link level detection which is not always possible), fast flooding, and more frequent shortest-path-first calculations. However, through analysis and simulation we show that this compromises the scalability and stability of the network. This is mainly because hello packets received at a router are indistinguishable from other packets and may experience long queueing delays during a sudden burst of many LSA (link-state advertisement) updates and cause the associated link to be declared down. At a later instant the link would recover. Both the failure and recovery generate fresh LSA, in effect developing a feedback loop to sustain the LSA storm. We suggest that hello and potentially some other IGP packets, such as acknowledgments, be marked explicitly so that they may be distinguished and queued separately from other IGP and data packets at the line rate and provided prioritized treatment. This allows significant reduction in IGP convergence time while maintaining network stability.