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The differences between OSPF and ISIS

Exemple OSPF topology with the backbone area and 2 stub areas
The most commonly used internal Gateway protocols (IGPs) in the Internet are OSPF and ISIS. Both are non proprietary and standardized in the IETF. Both are link state protocols. So, they have a lot of similarities (in fact, OSPF came last and largely reuses the principles of ISIS).

Many people ask for the differences between OSPF and ISIS in forums. As I have worked with both when I was a network architect, I thought that I could help you figure out the key differences between these two protocols.

OSPF runs on L3, i.e., IP ISIS runs on L2 (e.g., Ethernet)
OSPF v2 for IPv4, OSPF v3 for IPv6    ISIS supports IPv4 and IPv6 in a single protocol instance
OSPF is mainly used in enterprise networks ISIS is used by large internet service providers
OSPF requires area 0 (backbone) ISIS does not require area 0
IP makes OSPF slightly more vulnerable to attacks Running on L2 makes ISIS slightly safer
ISIS has the reputation to be slightly more scalable than OSPF

If you want to know more about the comparison of OSPF and ISIS, and their respectives advantages / drawbacks, there was a more comprehensive comparison at NANOG for a few years. You can find it here.


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