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If you can connect and authenticate but applications stall, time out, or fail to load your MTU (Maximum Transfer Unit) may be incorrect. If you are unable to connect to the VPN at all, please click here.

Troubleshooting Tip: If broadband users are having troubles with the VPN have them try to connect via dial up (if possible). If users can access the VPN correctly using dial up it is probably an MTU issue. Dial connections use a default MTU of 768 bytes.

The standard Ping Test is not the best way to determine your optimum MTU when using a VPN client. Although tedious, manually adjusting the MTU by trial and error testing is the best method. You will change the MTU and test the VPN for connectivity then adjust the MTU to the largest size possible that works. Important Note: You must lower the MTU on all client PCs as well as the VPN Server(s). Many client side applications automatically adjust the MTU automatically when it is installed on the end users PC, however the VPN server is commonly overlooked. Please reference the MTU troubleshooting article for a brief overview of the problem.

The MTU size of a network adapter can be changed in one of two ways:
    1) Locate the Windows registry key associated with the adapter and manually change the key.
    2) Use a third party application (such as DrTCP) with a GUI interface. Since there are so many variables with editing the registry this FAQ provides a step by step procedure using DrTCP.

Adjusting the MTU for a VPN using DrTCP

Step 1
Download DrTCP and open the application. Select the proper VPN network adapter from the pull down menu and change the MTU to 1400 bytes. Important Note: There may be more than one network adapter showing in the pull down menu for your PC. You must make sure you change the MTU on the correct network adapter associated with your VPN client. If there is any doubt as to which adapter is the correct one change the MTU on all adapters. Restart your PC. The changes will not be set unless your completely restarted your PC.


Picture by Andy Houtz


Step 2
After the PC has restarted, open a VPN session and test your connectivity to the server as well as any applications.
    If your VPN works correctly at 1400 bytes
    Repeat Steps 1 and 2 but increase your MTU to 1420 bytes. Continue to repeat Steps 1 and 2 (increasing your MTU by 20 bytes each time) until your VPN does not function correctly and back the MTU down to the last known fully functional MTU size. Remember you must restart the PC after every MTU change.

    If your VPN does not work correctly at 1400 bytes
    Repeat Steps 1 and 2 but decrease your MTU to 1380 bytes. Continue to repeat Steps 1 and 2 (dropping your MTU by 20 bytes each time) until your VPN functions correctly. Remember you must restart the PC after every MTU change.

Important Notes
  • Some Linksys router model/firmware have an "Auto" MTU capability that does not function correctly. You must enable the manual MTU function and adjust the size to 1492 (or less).
  • If you have a network with more than one computer, all NICs, adapters, and router(s) should have the same MTU setting. Please reference the links below to learn how to change the MTU on some popular routers:
  • MTU change on a Linksys Router
  • MTU change on a D-Link Router

    Andy Houtz DSL


  • Feedback received on this FAQ entry:
    • I have a pc using VPN that functions at an MTU of 1350 and several other pcs that don't use VPN that function at an MTU of 1500. Should I set the MTU on all my machines to 1350? pdeprez1@aol.com

      2008-08-15 01:44:14



    Expand got feedback?

    by Andy Houtz See Profile edited by FAQFixer See Profile
    last modified: 2008-08-13 15:41:02