In OSPF Hello process, if any of below mentioned item does not match, two routers do not form neighbor relationship:-
- Must Pass Authentication
- Must be in the same primary subnet, including subnet mask
- Must be in same OSPF Area
- Must be of same area type
- Must not have duplicate RIDs
- OSPF Hello and Dead timers must be equal
As all the mentioned parameters are exchanged and checked in Hello packets, if not matched, router will not become neighbors
Now the test scenario
1) Take a case of frame relay network
If we define interface type at one end as broadcast and other as point to point, as the default timers for both the interface type are same so neighbor relation should come up. But do they become adjacent.
I think they will become adjacent as interface type check is not done anytime during the OSPF adjacency process, just the related parameter i.e. timers are checked (that we made sure are equal)
neighbor status (show ip ospf neighbor) will be "Full/-" at P2P end and "Full/DROther? at broadcast defined interface. Broadcast will think its neighbor a DROther.
First question: - Is the above explanation correct?
Second question: - What are the operational issues with interface type mismatch even if both the router are adjacent now and why there are issues?
2) Similarly if we check the same scenario in an Ethernet network i.e. interface mismatch case with same default timer? Will the behavior be different from frame relay?
The two routers have exactly identical LSDB. It is just one router LSA sees the link between them has p2p and the other has broadcast. The SPF calculation on each router will therefore not use this perticular link.
Hope this helps,