<P> IS - IS was developed at roughly the same time that the Internet Engineering Task Force IETF was developing a similar protocol called OSPF . IS - IS was later extended to support routing of datagrams in the Internet Protocol (IP), the Network Layer protocol of the global Internet . This version of the IS - IS routing protocol was then called Integrated IS - IS (RFC 1195) </P> <P> Both IS - IS and Open Shortest Path First (OSPF) are link state protocols, and both use the same Dijkstra algorithm for computing the best path through the network . As a result, they are conceptually similar . Both support variable length subnet masks, can use multicast to discover neighboring routers using hello packets, and can support authentication of routing updates . </P> <P> While OSPF was natively built to route IP and is itself a Layer 3 protocol that runs on top of IP, IS - IS is an OSI Layer 2 protocol . It is at the same layer as Connectionless Network Protocol (CLNP). The widespread adoption of IP may have contributed to OSPF's popularity . IS - IS does not use IP to carry routing information messages . OSPF version 2, on the other hand, was designed for IPv4 . IS - IS is neutral regarding the type of network addresses for which it can route . This allowed IS - IS to be easily used to support IPv6 . To operate with IPv6 networks, the OSPF protocol was rewritten in OSPF v3 (as specified in RFC 2740). </P> <P> IS - IS routers build a topological representation of the network . This map indicates the subnets which each IS - IS router can reach, and the lowest - cost (shortest) path to a subnet is used to forward traffic . </P>

What allows the is-is routing protocol to operate on ipv6 networks
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