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Intermediate system to intermediate system (IS-IS), is an IGP routing protocol originally designed for CLNS as part of the OSI protocol stack and described in ISO 10589 . It was extended to include support for routing IP. It is now used in large backbone carriers today for delivering IP routing information. It maintains a link state database similar to OSPF. It supports variable length subnet masks. As with all routing protocols it resides at the network layer of the OSI model. IS-IS uses multicast to discover neighbouring routers using hello packets. It supports authentication of routing updates.