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@TECHREPORT{Varadhan96persistentroute,
    author = {Kannan Varadhan and Ramesh Govindan and Deborah Estrin},
    title = {Persistent route oscillations in inter-domain routing},
    institution = {Computer Networks},
    year = {1996}
}

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Abstract:

Hop-by-hop inter-domain routing protocols, such as BGP and IDRP, use independent route selection to realize domains' local policies. A domain chooses its routes based on path attributes present in a route. It is widely believed that these interdomain routing protocols always converge. We show that there exist domain policies that cause BGP/IDRP to exhibit persistent oscillations. In these oscillations, each domain repeatedly chooses a sequence of routes to a destination. Complex oscillation patterns can occur even in very simple topologies. We analyze the conditions for persistent route oscillations in a simple class of inter-domain topologies and policies. Using this analysis, we evaluate ways to prevent or avoid persistent oscillations in general topologies. We conclude that if a hop-by-hop inter-domain routing protocol allows unconstrained route selection at a domain, the protocol may be susceptible to route oscillations. Constraining route selection to a provably "safe " procedure (such as shortest path) can reduce the number of realizable policies. Alternatively, a routing policy registry can help detect unsafe policies. Explicit routing can then be used to realize desired policies (i.e., make routes available) that hop by hop routing cannot safely advertise. 1

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