IS-IS versus OSPF


IS-IS Protocol and OSPF have some similarities and differences. In this lesson, we will focus on IS-IS versus OSPF and we will learn the similarities and differences between ospf and IS-IS Protocol.


Lets firstly check the similarities of these two protocol one by one:

  • IS-IS Protocol and OSPF are both Link-State Interior Gateway Protocols.
  • Intermediate System to Intermediate System & OSPF are both uses Dijkstra SPF (Shortest Path First) algorithm.
  • These protocols have LSDBs and the LSDB similar structure
  • Both support VLSM (Variable Length Subnet Mask)
  • Both are very scalable protocols
  • Open Shortest Path First and ISIS have fast convergence time after changes
  • Both are open standards
  • Both have hierarchical structures, there are two hierarchy level
  • OSPF and Intermediate System to Intermediate System support LANs and point-to-point protocols in similar ways
  • Both protocol use periodic Hello messages for neighbour establishment


You can also check Cisco IS-IS Configuration Example.


After IS-IS versus OSPF similarities, now let’s check the differences of these two important protocols:


  • IS-IS Protocol works above Layer 2 next to IP, OSPF works on top of IP.
  • IS-IS routers belong to one area, OSPF routers belong multiple areas, links are in areas in Open Shortest Path First.
  • Intermediate System to Intermediate System uses DIS, OSPF uses DR and BDR.
  • OSPF has virtual-links, Intermediate System to Intermediate System does not support this.
  • Open Shortest Path First uses opaque LSA extensions, ISIS can be extended more by TLVs.
  • Open Shortest Path First has efficient packet types, ISIS has untidy structure because of TLVs, but more granular.
  • In OSPF small changes causes small packets, in Intermediate System to Intermediate System even if small change, entire LSPs are sent.


You can also see the summary of these differences in the shape.

IS-IS Protocol versus OSPF Protocol
IS-IS Protocol versus OSPF Protocol

And lastly, let’s check the terminology of these two Link-State protocols. IS-IS Protocol terminology versus OSPF terminolog is showed below:

IS-IS Protocol and OSPF Protocol Terminology Comparison
IS-IS Protocol and OSPF Protocol Terminology Comparison


Both of these routing protocols are ery important for networking world. So, it is important to know the differences and simiarities of these routing protocols.


Lesson tags: routing, is-is
Back to: CCIE Enterprise Infrastructure > IS-IS
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