MTU settings on Tunnel interface inaccurate

Unanswered Question
Jun 10th, 2009

We put our GRE tunnel interface to "ip mtu 1400". Yet a show interface tunnel says "MTU 1514 bytes, BW 4632..." The interface works fine, but out statistics are way, way off.

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Overall Rating: 4 (1 ratings)
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Richard Burts Thu, 06/11/2009 - 03:17

James

IP MTU is different from interface MTU and in GRE tunnels either (or both) can be set. So you could do something like this:

interface tunnel 0

interface mtu 1400

to make the interface MTU correspond to what you have set for the IP MTU.

HTH

Rick

jimmyc_2 Thu, 06/11/2009 - 05:46

Thanks Rick,

Do either of these commands have a physical impact on fragmentation, or are they just for statistics? As I mentioned, our tunnel interface statistics are 10x the physical interface statistics; how can I determine if the accounting is off or are just fragmenting packets needlessly? regards, jimmyc

Richard Burts Fri, 06/12/2009 - 07:07

James

I would expect that either of these commands would potentially have an impact on fragmentation.

HTH

Rick

jimmyc_2 Mon, 06/22/2009 - 13:01

Hi Rick,

We found the problem was asymmetric routing. I should have looked at the switch ports as well as the serial ports. I made an assumption that I shouldn't have. Thanks for your help, as always.

Jimmyc

Richard Burts Mon, 06/22/2009 - 13:46

Jimmyc

I am glad that you got it worked out. Thank you for posting back to the forum indicating that you had resolved the problem and what the problem was. It makes the forum more useful when people can read about a problem and can get confirmation about what the problem turned out to be.

HTH

Rick

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