OSPF and different MTU

[Konstantin Dunaev]

Hello,

I will now try to ask here

I have some problems with stability of my OSPF session between 3 devices. the topology is simple:

cisco7200 --- catalyst3560 --- cisco2600

the cisco7200 and catalyst3560 are speaking MTU 1546 (because of QinQ and MPLS)

cisco2600 speaks only MTU 1500.

The OSPF between cisco7200 and catalyst3560 works perfect, but the OSPF betwenn catalyst3560 and cisco2600 is not stable and doesn't come correctly to Full adjacency. The reason is quite stupid - catalyst3560 doesn't support differet MTU on different Interfaceses and tries to send OSPF packets with MTU 1546 to cisco2600, which obviously drop them.

The comman "ip ospf mtu-ignore" IS CONFIGURED on the both sides  but doesn't help, and I know that MTU value should be the same alone the  whole path, but cisco2600 is just a "spoke" router and doesn't forward  any backbone traffic and cisco2600 doesn't support lager MTU anyway.

What would be the best soluition or work-around? static routing is not really an option.

[lephuc007]

Hi Konstantin,

I appreciated of your feedback.Thank you

Regards,

Le

[Konstantin Dunaev]

Hello community,

as you can see it was a long story with a OSPF session between  C3560 and c2600 ,

but after some hours debuggin and analysing, I think I got the solution which could be helpfull for any others.

In short: C3560 with system MTU 1546 can't succesfully exchange the OSPF database information with c2600, because the router drops the packets which size is  lager as 1500 bytes, and if the OSPF database is quite large,  tries the c3560 to compose the OSPF packet upto  its system mtu limit - 1546 bytes.

And  again it's not the problem which are usually solved by "ip ospf mtu-ignore".

I had just noticed that OSPF adjacency between c3650 and c2600  goes UP and DOWN in a quite regular intervals - every 30 minutes. What happens in OSPF area every 30 minutes? - Exactly! The OSPF database flood.

It seems that during the initial OSPF establishing process can c3560 and c2600 somehow reach the FULL status on both sides, may be because c2600 just request every OSPF Database entry in a single LSA and not all together. But every 30 minutes   comes OSPF flood during which c3560 tries  the send a large OSPF update which is droped by c2600, and as с3650 doesn't get the ACLK back the adjecency is destried.

There is a magic interface  command "ip ospf flood-reduction" which solves this problem. After activation on the c3650  interface keep the OSPF session UP and running without any flapping.

[lephuc007]

Thank you Konstantin for the latest info.