This document describes extensions to the Intermediate System to Intermediate System (IS-IS) protocol to support optimal routing within a two-level domain. The IS-IS protocol is specified in ISO 10589@ with extensions for supporting IPv4 (Internet Protocol) specified in RFC 1195. This document replaces RFC 2966. This document extends the semantics presented in RFC 1195 so that a routing domain running with both level 1 and level 2 Intermediate Systems (IS) (routers) can distribute IP prefixes between level 1 and level 2@ and vice versa. This distribution requires certain restrictions to ensure that persistent forwarding loops do not form. The goal of this domain-wide prefix distribution is to increase the granularity of the routing information within the domain.