RE: OSPF MTU problem

From: samccie2004@yahoo.co.uk
Date: Sat Jul 03 2004 - 09:07:25 GMT-3


Are iu trunking , if so u might be adding more to MTU due to dot1q.

Just a guess, pls correct me if I am wrong.

Sam

-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
barrerj1@hotmail.com
Sent: 03 July 2004 13:26
To: ccielab@groupstudy.com
Subject: OSPF MTU problem

Hi all!

 

While doing an OSPF practice lab, I was encountered by the following
problem.

 

2 routers connected to a 3550 and running OSPF between them, the state
was stuck in EXSTART and couldn't complete the adjacency.

 

I checked CCO and found that the problem was an MTU mismatched.
However, the CCO doc points this problem to be a common when you connect
to another vendor's router. In my case all devices are CISCO.

 

 

 

Neighbors Stuck in Exstart/Exchange State

The problem occurs most frequently when attempting to run OSPF between a
Cisco router and another vendor's router. The problem occurs when the
maximum transmission unit (MTU) settings for neighboring router
interfaces don't match. If the router with the higher MTU sends a packet
larger that the MTU set on the neighboring router, the neighboring
router ignores the packet.

http://www.cisco.com/en/US/tech/tk365/tk480/technologies_tech_note09186a
0080093f0d.shtml

 

 

 

Can any one explain why this behavior as I did not change any of the MTU
configurations before the problem occurred?

 

 

Thanks

JB



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