OSPF Neighbor Adjacency works with Network Type Mismatch

In order for OSPF to work, the network type should be the same on both ends. Technically it's not required to form an OSPF adjacency because the network type is not exchanged.

The hello and dead interval have to match on both ends. The other criteria is that one network type requires a DR/BDR election and the other doesn't.

You can make the following network type mismatches work after making sure the hello and dead intervals are the same:

• Broadcast with NBMA : A broadcast network type can form an adjacency with an NBMA network. However, you need to manually specify neighbors on the NBMA side. The broadcast side will consider the NBMA side as a broadcast and will try to establish adjacency with it.
• Point-to-Multipoint with Broadcast/NBMA/P2P: Point-to-Multipoint can form an adjacency with any network type. However, on the P2MP side, you need to manually specify the neighbors.
• P2MP with P2MNB: P2MP can form an adjacency with P2MNB, but you’ll need to manually specify neighbors on both sides.
• P2P with any other type: Point-to-Point can form an adjacency with any other network type. However, manual neighbor specification may be required depending on the other network type.

Even though you might be able to establish a neighbor adjacency, you'll run into reachability issues because there might be discrepancies in the LSDB and SPF computation. That's especially true when you mix broadcast or non-broadcast with P2P or any of the P2MP network types.

The above information is nice to try in labs but it should never be used to create an OSPF topology in a production network.

It’s always best to avoid network type mismatches, as they can lead to unexpected behavior and complicate troubleshooting. Always ensure that both sides of a link are using the same network type.