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New Member

ospf packet length mismatch ?

m getting thousands of hits on IDS with ospf packey length mismatch between two cisco routers..please let me know how to stop this...

thanks in advance

regards..

manik...

6 REPLIES

Re: ospf packet length mismatch ?

Hello,

Two cisco routers will not become OSPF neighbors if the MTU's on the neighboring interfaces are not the same. Please make sure the MTU's on those interfaces are the same.

Kind Regards,

M.

New Member

Re: ospf packet length mismatch ?

hello...both the routers are connected through L3 switch.they are neighbours with each other..

Thanks..

Manik

Re: ospf packet length mismatch ?

Hi,

I do not think its a MTU issue.Because packet length says about the length of the ospf packet including header.

Check whether the configuration is same for both outers.Also see whetehr they are adjacent with each other.

HTH,

regards,

shri :)

Bronze

Re: ospf packet length mismatch ?

yes i support that the above post..

If ur ospf is working fine,i think it can happen any signature of low severity might be triggred in ids.can u check for the signature details of that event..

New Member

Re: ospf packet length mismatch ?

Can you please post errors your getting on IDS?

Cheers,

Nikhil E.

Re: ospf packet length mismatch ?

Are your devices forming adjacency or not?

-If the MTU mismatched the adjacency will never come up, unless manually configured to be matched on both ends on the link (neighbors) or ignore the MTU check during adjacency negotiations

Example:

Router(config-if)#int g0/0

Router(config-if)#ip ospf mtu-ignore

If your devices already forming adjacency and u only observe packet length mismatch, If you read section 8.2 of RFC 2328, there is no place that says

That an OSPF router should make this check, and the OSPF header

Length will always be different than IP length

http://www.faqs.org/rfcs/rfc2328.html

Best Regards,

Mounir Mohamed

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