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IS-IS (pronounced eye-ess-eye-ess) is defined in ISO 10589. IS-IS is the descendant of the older OSI CLNS routing protocol. IS-IS is a dynamic , link state routing protocol. In an IS-IS network, there are End Systems, Intermediate Systems, Areas and Domains. End systems are user devices. Intermediate systems are routers. Routers are organized into local groups called 'areas' and several areas are grouped together into a 'domain'. IS-IS places Area routers into Layer 1, and routers that interconnect the Areas into Layer 2. IS-IS uses it's own addressing scheme.

There are two types of IS-IS addresses:

  1. Network Service Access Point (NSAP)
    NSAP addresses identify network layer services, one for each service running. (NSAP's are broken down as follows)
    • Initial Domain Part (IDP)
      • Authority and Format Identifier (AFI) (1-byte)
        Provides information about the structure and content of the IDI and DSP fields.
      • Initial Domain Identifier (IDI) (variable length)
    • Domain Specific Part (DSP)
      • High Order Domain Specific Part
      • Area ID
      • System ID
      • n-selector
        (1-byte value that serves a function similar to the port number in Internet Protocol).
  2. Network Entity Title (NET)
    NET addresses identifynetwork layer entities or processes instead of services.
Devices may have more than one of each of the two types of addresses. However NET's should be unique, and the System ID portion of the NSAP must be unique for each system.

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