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(also Intermediate System-to-Intermediate System)

IS-IS definition

Intermediate System-to-Intermediate System (IS-IS) is a link-state routing protocol primarily used within large networks such as ISPs or enterprise networks. It enables routers to exchange routing information for the purpose of determining the most efficient path for forwarding packets across the network. IS-IS operates on the OSI model's network layer (Layer 3) and is independent of any specific network protocol, making it highly adaptable to various network environments.

See also: Network layer, Routing table

IS-IS comparison to similar protocols

OSPF (Open Shortest Path First): OSPF is another widely used link-state routing protocol but operates solely on IP networks. While both IS-IS and OSPF provide efficient routing and fast convergence, IS-IS offers greater flexibility due to its network protocol independence.

IS-IS pros and cons


  • Scalable. IS-IS is well-suited for large networks and can handle a large number of routers.
  • Protocol-agnostic. It is independent of any specific network protocol, providing adaptability to different environments.


  • Complexity. IS-IS can be more challenging to configure and manage compared to other routing protocols like OSPF.

IS-IS tips:

  • Ensure that you have a thorough understanding of the OSI model and network layer concepts before configuring IS-IS.
  • In networks with both IPv4 and IPv6, consider using IS-IS because it can handle both protocols simultaneously.