IS-IS Overview


IS-IS is a protocol widely used within Service Provider networks. It was originally developed for routing of CLNP protocol.

IS IS Overview

  • Integrated ISIS refers to ISIS operating for IP protocol and CLNP (two different network layer protocols).
  • ISIS runs directly on the Data Link header, unlike OSPF that uses IP.
  • Addressing of node is based on CLNP Network Service Access Point (NSAP).
  • Areas are designated in two hierarchies: Level-1 and Level-2.
    • Level-1-local route exchange
    • Level-2-backbone route exchange
  • Routers have the area membership defined and not interfaces as in OSPF.
  • Protocol Data Units (PDU) are was CLNP uses as packets.
  • Three categories of PDU:
    • Hello - Establish adj and maintain it.
    • Link-state - Distribute routing information.
    • Sequence - Synchronization for Link-State.
  • ISIS has two network types used to establish adj over different link types:
    • Point to Point
    • Broadcast
  • There are three type of adjacencies:
    • Level 1
    • Level 2
    • Level 1-2


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