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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 from Cisco for a brief overview of the problem. Important Note: You must lower the MTU on all client PCs as well as the VPN Server(s).
DrTCP can be used to lower the MTU on almost any client or server available and a step by step guide is shown below.
Download DrTCP to all VPN enabled PCs/Servers 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. You must make sure you change the MTU on the correct network adapter associated with your VPN. If there is any doubt as to which adapter is the correct one change the MTU on all adapters. Restart the PC or Server. The changes will not be set unless your completely restarted your PC.
Picture by Andy Houtz
After the PC has restarted, open a VPN session and test your connectivity to the server as well as any applications.
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/Server 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 (decreasing your MTU by 20 bytes each time) until your VPN functions correctly. Remember you must restart the PC/Server after every MTU change.
•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, servers, and router(s) should have the same MTU setting. Reference the links below for related links:
•MTU change on a Linksys Router
•MTU change on a D-Link Router
•MTU change on a Cisco 837 Router
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