Avoiding Ties in Shortest Path First Routing

First we discuss problems associated with ties and flow splitting with shortest path first protocols such as OSPF and IS-IS. The problems relate to uncertainty in the splitting when there are multiple shortest path from a source to a destination. Even if routers are configured for even splitting, there can easily be unpredicted biases that can overload links and thereby affect quality of service guarantees for virtual leased lines. Second we show how one can set the OSPF/IS-IS weights so as to avoid ties and yet minimize congestion. On real and synthetic networks we demonstrate experimentally that load balancing typically can be done nearly as well without ties as with ties assuming exact even splitting. In fact we get close to the optimum for general routing, including the possibilities with MPLS. The contribution of the second author is an appendix with measurements from a real network showing how even spltting can be off by 20%. Such bias can replicate if the traffic meets multiple ties from source to destination. Keywords— SPF, OSPF, IS-IS, traffic engineering, traffic management, local search, combinatorial optimization.