area --A physically connected portion of a routing domain in which all routers are assigned a common area address. Also known as the Level-1 subdomain. A routing domain may consist of multiple areas that are reachable by traversing the Level-2 subdomain.
area address --The high-order octets of the Network Entity Title (NET) assigned to an IS. All ISs in the same Level-1 area are assigned the same area address.
CLNP --ISO Connectionless Network Protocol as defined in ISO 8473.
DIS --Designated Intermediate System. An IS elected by all the ISs operating on a multiaccess circuit at a given level to represent the multiaccess circuit. The DIS sends pseudonode LSPs on behalf of the circuit advertising adjacencies to all the ISs operating on that circuit.
domain --The portion of a network on which the IS-IS protocol is configured to operate. The routing domain consists of all Level-1 areas and the Level-2 subdomain.
ES --end system. An ES is any nonrouting host or node.
Integrated IS-IS --Extended form of IS-IS that supports multiple network protocols. Extensions have been defined in IETF documents, especially RFC 1195.
IS --intermediate system. OSI term for a router.
IP --Internet Protocol Version 4, also known as IPv4.
IPv6 --Internet Protocol Version 6.
IS-IS --Intermediate System-to-Intermediate System. Routing protocol as defined in ISO/IEC 10589.
Level-1 router --An IS that supports Level-1 routing for its assigned area.
Level-2 router --An IS that supports Level-2 routing.
Level-2 subdomain --All Level-2 capable routers in a domain and the links that interconnect them. Level-1 areas are interconnected via the Level-2 subdomain. For routing in a domain to work properly, the Level-2 subdomain must not be partitioned.
NET --Network Entity Title. An address assigned to an instance of the IS-IS protocol. The NET includes an area address, a system ID, and an N-selector. When multiple NETs are assigned to an IS-IS instance, only the area address portion of the NET may differ.
NSEL --N-selector. The least significant octet of a Network Entity Title. It is always assigned the value 00.
system ID --The part of the NET that immediately follows the area address. The field is 6 octets long.