This paper proposes an architecture of Intermediate System-to-Intermediate System (IS-IS) for the ATM based Multiprotocol Label Switching (MPLS) system. IS-IS is a link state routing protocol designed to provide routing in a network layer protocols with datagram service. IS-IS has favored scalability in the aspect of minimizing storage and computing in level 1 routers. Therefore, it is important to support IS-IS for the MPLS system used in backbone networks. We propose the architecture of IS-IS routing protocol and extensions for traffic engineering in MPLS system. Also, we describe a packet transmission scheme and a configuration procedure.