Intermediate System-to-Intermediate System (IS-IS) is an International Organization for Standardization (ISO) dynamic routing specification. IS-IS is described in ISO/IEC 10589, reprinted by the Internet Engineering Task Force (IETF) as RFC 1195. IS-IS is a link-state routing protocol that floods link-state information throughout the network to build a picture of network topology. IS-IS was primarily intended for routing OSI Connectionless Network Protocol (CNLP) packets, but it has the capability to route IP packets. IP packet routing uses Integrated IS-IS, which provides the capability to route protocols such as IP.
IS-IS is a common alternative to other powerful routing protocols such as OSPF and EIGRP in large networks. Although not seen much in enterprise networks, IS-IS is commonly used for internal routing in large ISP networks. IS-IS is also getting more use in data center technologies such as Overlay Transport Virtualization (OTV) and fabric path. As with OSPF, IS-IS uses the Dijkstra algorithm to calculate the shortest-path tree (SPT); it also uses link-state packets (LSPs) instead of OSPF link-state advertisements (LSAs). Also, neither OSPF nor IS-IS are proprietary protocols.
IS-IS creates two levels of hierarchy, with Level 1 for intra-area routing and Level 2 for interarea routing. IS-IS distinguishes between Level 1 and Level 2 intermediate systems (ISs). Level 1 ISs communicate with other Level 1 ISs in the same area. Level 2 ISs route between Level 1 areas and form an interarea routing backbone. Hierarchical routing simplifies backbone design because Level 1 ISs only need to know how to get to the nearest Level 2 IS.
Note
In IS-IS, a router is usually the IS, and personal computers, workstations, and servers are end systems (ESs). ES-to-IS links are Level 0.
IS-IS Metrics
IS-IS, as originally defined, uses a composite metric with a maximum path value of 1024. The required default metric is arbitrary and is typically assigned by a network administrator. By convention, it is intended to measure the capacity of the circuit for handling traffic, such as its throughput in bits per second. Higher values indicate lower capacity. Any single link can have a maximum value of 64. IS-IS calculates path values by summing link values. The standard sets the maximum metric values to provide the granularity to support various link types, while ensuring that the shortest-path algorithm used for route computation is reasonably efficient.
In Cisco routers, all active interfaces have a default metric of 10. If an interface is passive, the default value is 0. The administrator must configure the interface metric to get a different value. This small metric value range has proved insufficient for large networks and provides too little granularity for new features such as traffic engineering and other applications, especially with high-bandwidth links. Cisco IOS software addresses this issue with the support of a 24-bit metric field, the “wide metric.” Wide metrics are also required for route leaking. Using the new metric style, link metrics now have a maximum value of 16,777,215 (224 − 1) with a total path metric of 4,261,412,864 (254 × 224 = 232). Deploying IS-IS in the IP network with wide metrics is recommended for enabling finer granularity and supporting future applications such as traffic engineering.
IS-IS also defines three optional metrics (costs): delay, expense, and error. Cisco routers do not support the three optional metrics. The wide metric noted earlier uses the octets reserved for these metrics.