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IS-IS Network Design Solutions by Scott Sturgess, Abe Martey

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Calculating IS-IS Routes with the SPF Algorithm

Annex C2 of ISO 10589 specifies use of the SPF algorithm for route calculation within the IS-IS protocol. Annex C of RFC 1195 specifies modifications of the SPF algorithm for supporting IP routing within the IS-IS protocol. Dijkstra's algorithm creates a shortest path tree of the network topology, from which the shortest (best) paths to various destinations in the network are determined and entered into the IP routing table. To obtain best paths for IP routes, IP subnets are considered as leaves in the shortest path tree. Therefore, network events resulting in changes in only IP reachability entries contained in link-state packets do not require computation of the entire shortest path tree. In such ...

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