Choosing a routing protocol to implement in your network is not an easy task, yes the chosen routing protocol would eat the resources or would keep it free for the best performance and choosing a routing protocol mainly depends on the size and the available resources of the network.

In service provider’s networks we don’t have many choices but the challenge is so tough between both known protocols “ISIS and OSPF”.

We know that ISIS and OSPF have been published at the early days of networking, and the internet battle ended by adopting IP than ISO to be the standard of the internet, respectively OSPF was adopted over ISIS as long as ISIS was depending on ISO addressing and standards. Days after, ISIS has been developed to be integrated with IP suit and got the chance again to enter a new battle against OSPF.

At the old days, we had no choice and implementing OSPF in all SP networks, but NOW we have also ISIS and many scouts are wondering which protocol is better to implement and what are the differences?

Yes, good question “What are the differences between ISIS & OSPF” that really would help in choosing an IGP for your SP network.

Both ISIS & OSPF are link-state protocols and working with the concept of AREAs and all routers within the same area should have an identical database.

Below is a table showing the similarities and the differences between ISIS & OSPF:




Link State

Link State





Contiguous chain of Level-2 routers and could be in different areas

Area “0” connected to all areas

Area types








Within separate instance OSPFv3








As you see you won’t find many differences between ISIS & OSPF, just ISIS can support IPv6 within the same instance and on the other side OSPFv3 is mandatory for IPv6 support, also what makes ISIS is so flexible protocol is using TLVs “Type, Length & Value” that makes ISIS open for any new services or extensions, read more about ISIS TLV concept

There are some differences in the operation itself, like Designated Router operation & MTU mismatch discovery. OSPF will discover the MTU mismatch in a late step during exchanging the database between two neighbors so that one of them will receive a DBD packet and the value of the MTU field exceeds what is locally configured so that the DBD will be dropped and neighbors will not form a full adjacency unless this router is configured to ignore the MTU field. Unlike OSPF, ISIS will discover the MTU mismatch at the beginning of the 1st hello as long as ISIS pads the hello message until the size reaches the outgoing interface’s MTU so that if there is a mismatch, neighbor will not receive neither a hello nor ACKs and ISIS has nothing to do with that just keep calm with no established adjacency.

OSPF MTU discovery operation disadvantage it may waste the resources of the link and the router itself as this is assumed as a late discovery and both neighbors will still trying to exchange the databases, but on the other hand it is simple in troubleshooting when seeing the neighborship stuck in EXSTRT state so the 1st common issue to check is the MTU configured on both neighbors.

ISIS MTU discovery operation disadvantage unlike OSPF, there is no sign or indication says “HEY, CHECK THE MTU” , but the good thing is that MTU mismatch discovery is done at the 1st stage before establishing any neighborship nor exchanging the database.

Note: Cisco and Juniper implement ISIS in different ways to avoid useless padding after forming the neighborship and that helps a lot to avoid eating link resources.

I don’t have much more to say about OSPF & ISIS just that ISIS is strongly recommended by all vendors to be used in SP networks as IGP.

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One thought on “ISIS vs OSPF

  1. Another 2 differences between ISIS vs OSPF are that ISIS can implement the authentication between the areas and OSPF cannot. Also ISIS can ignore the TLV that doesn’t understand in order to make the extendability and compatibility easier, OSPF cannot ignore the LSAs.

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