Protocols ISIS VS. OSPF

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L2 routers are similar to OSPF backbone routers, and the set of L2 routers (including L1/L2 routers) and their interconnecting links make up the IS-IS backbone, similar to area 0 in an OSPF network.

• IS-IS does not use level 3 routers.
• Some routers, called L1/L2 routers, belong to both area types.
• Unlike OSPF, IS-IS routers are not required to be connected to a contiguous backbone area. In fact, the backbone area can also be segmented in IS-IS.
• IS-IS uses the concepts of router levels, which is similar to OSPF areas. L2 routers are similar to OSPF backbone routers, and L1/L2 routers are analogous to OSPF ABRs.
• With IS-IS, there is no restriction that all backbone routers (level 2 routers) be contiguous
  such as the backbone area of OSPF.
  
• In OSPF all areas must be directly linked to area 0, and the backbone area must also not be segmented.
  
• With IS-IS, the backbone area can be more easily extended since all L2 routers need not be linked directly together.
  
• With regard to CPU use and the processing of routing updates, IS-IS is more efficient
  than OSPF.
  
• In IS-IS, one LSP is sent per IS-IS router in each area (including redistributed prefixes [routes]), compared to the many OSPF LSAs that would be sent.
  
• Not only are there fewer LSPs to process, but the mechanism by which IS-IS installs and withdraws prefixes is less processor intensive.
  
• In IS-IS, the entire SPF table is not refreshed periodically like OSPF, which does so every 30 minutes by default.

Reference: Jeff Doyle, “Routing TCP IP, Volume 1”