|Exemple OSPF topology with the backbone area and 2 stub areas|
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.