OSPF IS-IS comparison

The following is a comparison of the OSPF and IS-IS routing protocols:


  • Link state protocols
  • Use the Dijkstra algorithm for computing the best path
  • Support variable-length subnet masks
  • Use multicast to discover neighboring routers
  • Support authentication of routing updates
  • Build a topological representation of the network



  • is natively built to initially route IPv4 and later IPv6
  • is a Layer 3 protocol that runs on top of IP
  • uses IP to relay OSPF messages
  • employs areas
  • ABR routers can be in multiple areas, thus the border between areas is in the router itself
  • requires an area 0 backbone
  • has a larger set of extensions and optional features specified in the protocol standards


  • is an OSI Layer 2 protocol
  • does not use IP to carry routing information messages
  • employs the use of Level 1 (intra-area), Level 2 (inter-area) or Level 1-2 (both) routers
  • area borders exist between routers designated as Level 2 or Level 1-2
  • does not require a backbone area
  • fewer extensions and options, but more scalable and expandable




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