Setting MTU Value

[Chips]

I have read many messages concerning setting the MTU value have a
couple of different questions.

1. When trying to find the best MTU value for on laptop (W2K Pro) on
our network, I have found the value to be less than 1400. Everything
above 1345 causes the packets to fragment. For other systems on our
network, the MTU value is 1472 before the packets start fragmenting
(which is about the number I would expect). The VPN performance at
1345 over cable is very poor for this user and I would hope to bump
this up so the performance is better.

Why would this one system start fragmenting at such a low value while
everything I have read states the best MTU value should be between
1400-1500.

2. What it the correct registry key to set the MTU value.
Microsoft's page: http://support.microsoft.com/default.aspx?scid=kb;en-us;826159&Product=win2000
points to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NdisWan\Parameters\0
as the correct key with the parameter TunnelMTU to set while other
links in this group points to
HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\
and the parameter MTU to set.

Thanks,

Chips

 

[J.C. Hornbeck [MSFT]]

The system isn't causing fragmentation, the intermediate devices are.
Article 826159 works for VPN connections but if you want to change MTU for a
specific NIC then follow 'Method 3' in this article:

314825 - How to Troubleshoot Black Hole Router Issues
(http://support.microsoft.com/?id=314825).

--
J.C. Hornbeck, MCSE
Microsoft Product Support

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[Chips Fowler]

I agree that an intermediate device should be causing this but if this
system is connected to the same device as other systems that fragment at a
higher MTU value what does that mean?

-chips

 

[Chips Fowler]

I found a setting that seemed to fix my MTU problem. The paramter is
GlobalMaxTcpWindowSize found at

HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\

and set to 65535 seemed to allow me to increase my MTU value to what
eveybody else on the network has.

Related link of useful information:
http://cable-dsl.home.att.net/#IncreasingWindow
http://www.dslreports.com/

And thread found at google
http://groups.google.com/groups?hl=...-8&selm=OMnvmcqVBHA.1936%40tkmsftngp07&rnum=5

Thanks,

-chips