Things that must match in order to form OSPF Adjacency

This is going to be a real short post but still good to know and important too :-)


Here is quick list of things that must match between two OSPF Neighbors in order to form adjacency.

1. Area-ID must match.
2. Subnet Mask of interface (Except For Point-To-Point 
     interface types & Virtual-Links)
3. Authentication Type & Authentication Password
4. Stub Flag (AKA E-Bit)
5. Timers (Hello & Dead)
6. Compatible Network Types ( Broadcast Vs Non-Broadcast / P2P 
    Vs P2M Vs P2M non- broadcast)
7. MTU
8. Capability LLS (Could Be) 
9. Passive Interface


Note:- Although MTU is not part of requirements in General but it must match in order to exchange database. You can however make router not to consider it as part of negotiation by using command " ip ospf mtu-ignore". Command only needs to put on side with lower MTU supported.  Another cool new way to do it is  " system mtu routing   " for Catalyst Switches without changing System MTU for rest of operations but just for Routing Operations.


HTH...
Deepak Arora