For an OSPF adjacency to form some parameters must match:
- Belong to same IP network
- Subnet mask
- Area
- Authentication
- MTU
- If area is stub/NSSA
- Timers
So why is it a requirement for the MTU to match? Imagine that Router A has a MTU of 1500 bytes, Router B has a MTU of 9000 bytes. Depending on the number of links in the network it is possible that Router B would send router LSA or LSU that will exceed 1500 bytes.
From RFC 2328:
If the Interface MTU field in the Database Description packet
indicates an IP datagram size that is larger than the router can
accept on the receiving interface without fragmentation, the
Database Description packet is rejected.
If the MTU does not match the adjacency will not form. This OSPF provides a check for both data plane and control plane. By ensuring MTU is same the path will be able to be used by end hosts with a consistent MTU. Normally Path MTU Discovery (PMTUD) should be used for hosts to provide this check and avoid fragmentation but sometimes ICMP may be filtered.
From control plane perspective it keeps OSPF from breaking in such situations where a router would send larger LSA/LSU than the neighbor can accept. So if Router B sends LSA/LSU that is for example 3000 bytes. Then Router A can not accept this packet which means that synchronization can't occur. If you configure ip ospf mtu-ignore what happens is that OSPF ignores MTU as part of the state matchine checking if to form adjacency. The adjacency will then form but there may be other issues later. Many people believe that this is the solution but it may still be an issue when synchronizing the database.
The proper fix is to ensure that MTU is consistent between adjacent routers.
For more information see this link:
http://blog.ine.com/2011/03/30/ospf-and-mtu-mismatch/
Daniel Dib
CCIE #37149
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Daniel Dib
CCIE #37149
CCDE #20160011
Please rate helpful posts.
