IETF RFC 5120 PDF

This document describes an optional mechanism within Intermediate System to Intermediate Systems (IS-ISs) used today by many ISPs for IGP routing within their clouds. This document describes how to run, within a single IS-IS domain, a set of independent IP topologies that we call Multi-Topologies (MTs). This MT extension can be used for a variety of purposes, such as an in-band management network "on top" of the original IGP topology, maintaining separate IGP routing domains for isolated multicast or IPv6 islands within the backbone, or forcing a subset of an address space to follow a different topology.