OSPF stuck in exchange / Vlan Trunk

[yvanderunes802438600]

Hello,

I have two routers connected with multiple switchs beetween.

I'm encountering a problem for forming adjencies in OSPF.

I don't understand something, all my interfaces are with 1500 mtu value. 

But when i put a link beetween two switchs in trunk mode, the ospf can't be full state. If i put the link in access mode it's ok. 

I tried to set ip ospf mtu-ignored on the router and its fine.

In the DBD the value is 1500 on the both side, i have no message like MTU missmatch in the ospf events.

I dont understand why its a problem, for forming adjencies. The tagging add 4 bytes in the ethernet frame, but why there is an impact ? More over, not all the ports are in trunk mode on the path.

Regards

[MHM Cisco World]

Share 

Show ip ospf traffic <<- both peers

Also

Debug ip ospf adj <<- both peer

MHM

[M02@rt37]

Hello @yvanderunes802438600 

Very interesting use case.

Please note that, when a port is in 802.1Q trunk mode, each ethernet frame get a 4 byte vlan tag inserted. So, this increase the frame size from 1514 to 1518. Even if both router show mtu of 1500 and no mismatch is logged, L2 switch silently dropping or modifying DBD packets can cause the OSPF state machine to get stuck.

On the other hand, access ports do not add a vlan tag to the ethernet frame so frames size remains 1514 byte or less. There is no overhead, and the switches forward frames without concern and DBD packets make it through fully intact !

The command ip ospf mtu ignore "hide" that problem, rather than fixing this underlying MTU mismatch.

[M02@rt37]

Some sources:

https://networkengineering.stackexchange.com/questions/2598/ospf-stuck-in-exstart-adjacency-state

https://community.cisco.com/t5/service-providers-knowledge-base/ospf-and-mtu/ta-p/3118885

[paul driver]

Hello
Note_ the DBD packets are ignored if there is a mtu mismatch

Appending the ip ospf mtu-ignore can work in certain situations however its a band aid, basically you have a mtu issue on the network that needs to be address.

Also having two rtrs form a ospf adjacency between two switches should not cause an mtu issue tbh, can you confirm how the switch cfg'd, are there any layer 3 interfaces?