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IS-IS versus OSPF

IS-IS Protocol and OSPF have some similarities and differences.

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

– IS-IS Protocol and OSPF are both Link-State Interior Gateway Protocols.
– IS-IS Protocol and 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
– Both has a fast convergence time after changes
– Both are open standards
– Both have hierarchical structures, there are two hierarchy level
– Both protocols support LANs and point-to-point protocols in similar ways
– Both protocol use periodic Hello messages for neighbour establishment

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 Protocol routers belong to one area, OSPF routers belong multiple areas, links are in areas in OSPF
– IS-IS Protocol uses DIS, OSPF uses DR and BDR,
– OSPF has virtual-links, ISIS does not support this,
– OSPF uses opaque LSA extensions, ISIS can be extended more by TLVs
– OSPF has efficient packet types, ISIS has untidy structure because of TLVs, but more granular,
– In OSPF small changes causes small packets, in ISIS 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 protocol. 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

The other articles on IS-IS Protocol are below:

Routing With IS-IS Protocol – Part 1
Routing With IS-IS Protocol – Part 2 (IS-IS Addresses)
Routing With IS-IS Protocol – Part 3 (IS-IS Packet Types)
Routing With IS-IS Protocol – Part 4 (IS-IS Adjacency)
Routing With IS-IS Protocol – Part 5 (IS-IS versus OSPF)

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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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