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MTU question

Posted on 2004-03-24
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Last Modified: 2010-04-11
There is a device on my network using a 1300 mtu.  How can I find out which device it is?
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Question by:skippyllh
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Expert Comment

by:RDAdams
ID: 10671711
What type of wireless equipment are you running?  How many of them do you have.  Check wireless routers etc.
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by:RDAdams
ID: 10671718
If you have a large network it helps to diagram out each segment.
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by:skippyllh
ID: 10671809
There is no wireless.  My network is pretty small.
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by:lrmoore
ID: 10672278
First, how did you determine that there was a device with the MTU setting of 1300?
1300 is a default setting for any system with Cisco VPn client software..
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Author Comment

by:skippyllh
ID: 10672315
It was determined by a cisco engineer.  I have a client using my vpn concentrator to access a reporting server.  They have an app that forces them to only use 1500.  They cannot negotiate.  When they try to access my reporting server over a Lan-to-Lan tunnel (pix to 3030 concentrator) the software times out.  I opened a case up with tac and was told after they viewed all of the logs that i have a device in my network that is forcing the mtu to 1300.  it didn't make a whole lot of sense to me and I don't know how to determine the mtu of all of the devices on my network.  (without a lot of manual labor)
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Expert Comment

by:lrmoore
ID: 10672419
You only need to look at the client, the server, and all devices between the client and the server.

Client -->PIX-->router<---internet--->router-->VPN3030-->switch?-->server

Each interface of each device in between has the ability to be manually adjusted to specify an MTU
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Author Comment

by:skippyllh
ID: 10672545
I've looked at all of those.  There is still an issue.  According to the engineer at tac there could still be a device other than those causing this.  I'm just looking for an automated way to determine the MTU of the devices on my network.
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Accepted Solution

by:
blackss earned 68 total points
ID: 10673645
Use network monitor from a W2k server and look for the MTU size you want, or use another network sniffer...find your MTU and look at the IP and u have your culprit.
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by:Jman8R
Jman8R earned 66 total points
ID: 10673764
If you have a small network, you can check the MTU settings for each PC ( depending on the OS ) through the registry.

See: http://www.winguides.com/registry/display.php/280/

Although, I have seen cases where Cisco equipment themselves force MTU's down to 1300. This, however, was necessary due to the protocol being used over the ADSL connection.

As said in a previous post... The Cisco VPN software sets the above registry key so that the MTU is 1300. Maybe if you know of one of the clients that has that software installed, it would be a good starting point!
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by:Bing CISM / CISSP
Bing CISM / CISSP earned 66 total points
ID: 10683691
>> There is a device on my network using a 1300 mtu.  How can I find out which device it is?

umm... can you ping the device? if so, why dont you just ping it then use "arp -a" to get its MAC address which commonly gives general information of this device, such as brandname. of course, you should make sure the ping client should stay in same broadcast domain where the device resides in.
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