Most people probably know this but I ran across this information on TechRepublic a few months ago on determining the ideal MTU size and was surprised to see the results when I followed the instructions and tested some connections. Thought I would pass it on for what its worth.
-Dave Vantine >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>> Adjusting the Maximum Transmission Unit size Of all the registry tweaks that I've found for TCP/IP, perhaps the most useful is the ability to adjust the Maximum Transmission Unit (MTU) size. MTU size dictates the maximum packet size (in bytes) that the TCP/IP transport will attempt to transmit over the underlying network. Networks that use Internet Connection Sharing (ICS) or Point-to-Point Protocol over Ethernet (PPPoE) for their broadband connections must have an MTU size of 1,492 or lower to run efficiently. An incorrect MTU size can make your network run slowly. Administrators tend to dismiss this problem as low bandwidth on the ISP's end, and that might be a source of the problem. But you'd be surprised at the difference an MTU adjustment can make. The MTU is such a factor because, by default, Windows 2000 and Windows XP set the MTU size to 1,500. It's been my experience that if a network is running slowly, even the 1,492 value I mentioned earlier is too high. I usually have the best luck by setting the MTU to around 1,454. You can check the MTU size for your environment by opening a command prompt on your administration workstation and typing PING -F -I <MTUsize> <gateway> where MTUsize is the size you want to experiment with and gateway is the gateway of your network. Start out by using an MTU size of 1,454. Doing so will make the command look something like this: PING -F -I 1454 147.100.100.50 When you enter the command, the PING will either be successful or it will fail with a message stating that the data must be fragmented. If you receive the error message, decrease the MTU value and keep trying until you find a value that works. If the PING command works the first time, try incrementing the value by five or 10 until you see the error message. You can then narrow down to the appropriate value. Once you've discovered an MTU size that works, the next trick is to embed the value into the registry. The key's actual location within the registry varies depending on the version of Windows you're using. If you're using Windows NT, the MTU key is located at: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services \Parameters\Tcpip If you're using Windows 2000 or higher, the MTU key is located at: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services \Tcpip\Parameters\Interfaces\adapter ID There's a good chance that the MTU key doesn't presently exist in your system and that you'll have to create it at the specified location. To do so, open the Registry Editor and create a new DWORD value called MTU. The minimum value for this key is 68, and the maximum value is the MTU for the underlying network. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>> -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Horner, Josh Sent: Thursday, May 20, 2004 1:38 PM To: Exchange Discussions Subject: RE: Latency Issue Exch03 - Outlook The VPN is most likely using an MTU Size Outlook doesn't like. This might be useful. http://support.microsoft.com/default.aspx?scid=kb;EN-US;329918 Josh -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of David, Andy Sent: Thursday, May 20, 2004 1:11 PM To: Exchange Discussions Subject: RE: Latency Issue Exch03 - Outlook MTU size is often a culprit with VPNs. -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Crista Murphy Sent: Thursday, May 20, 2004 1:12 PM To: Exchange Discussions Subject: Latency Issue Exch03 - Outlook We have a large WAN with remote offices connecting to the exch03 office via PIX 501 to Sonicwall VPN. We are noticing severe latency problems in 90% of our remote offices when trying to use Outlook (2k std & pro and XP) which has shown up in conjunction with the move from the PIX to PIX VPN to the current configuration. Anyone have any suggestions on which diag logging to turn up on the exch server that might help diagnose this problem? The people in charge of the vpn's are 100% blaming the exch server for dropping sessions. Exch works fine in the local office and via webmail, it is only when connecting thru the VPN with Outlook. Thanks! _________________________________________________________________ List posting FAQ: http://www.swinc.com/resource/exch_faq.htm Web Interface: http://intm-dl.sparklist.com/cgi-bin/lyris.pl?enter=exchange&text_mode=&; lang =english To unsubscribe send a blank email to %%email.unsub%% Exchange List admin: [EMAIL PROTECTED] To unsubscribe via postal mail, please contact us at: Jupitermedia Corp. 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