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Abstract-What does a child's search of a large, complex cartoon for the eponymous character (Waldo) have to do with Internet routing? Network operators also search complex datasets, but Waldo is the least of their worries. Routing oscillation is a much greater concern. Networks can be designed to avoid routing oscillation, but the approaches so far proposed unnecessarily reduce the configuration flexibility. More importantly, apparently minor changes to a configuration can lead to instability. Verification of network stability is therefore an important task, but unlike the child's search, this problem is NP hard. Until now, no practical method was available for large networks. In this paper, we present an efficient algorithm for proving stability of iBGP, or finding the potential oscillatory modes, and demonstrate its efficacy by applying it to the iBGP configuration of a large Tier-2 AS.

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Routing oscillation is highly detrimental. It can decrease performance and lead to a high level of update churn placing unnecessary workload on router the problem is distributed between many providers. However, iBGP -the routing protocol used to distribute routes inside a single Autonomous System -has also been shown to oscillate. Despite the fact that iBGP is configured by a single provider according to apparently straight forward rules, more than eight years of research has not solved the problem of iBGP oscillation. Various solutions have been proposed but they all lack critical features: either they are complicated to implement, restrict routing flexibility, or lack guarantees of stability. In this paper we propose a very simple adaptation to the BGP decision process. Despite its simplicity and negligible cost we prove algebraically that it prevents iBGP oscillation. We extend the idea to provide routing flexibility, such as respecting the MED attribute, without sacrificing network stability.