From CT3

IS-IS is an interior gateway protocol (IGP) defined in ISO/IEC 10589:2002 (and republished in RFC 1142) as part of the ISO/OSI protocol suite. It's a link-state routing protocol (like OSPF), but can support multiple address families (CLNS, IP and IPv6) using the same topology database.

IS-IS is commonly used in very large IP networks because its implementation in Cisco IOS historically scaled better than OSPF. It's also the only choice when the network has to support CLNS-based network management.