Change MTU setting to fix connectivity issues

If you are facing any of the following …

• Speed issues
• Not able to open certain websites or connect to instant messengers
• VPN disconnects frequently

The issue is most likely related to MTU. Maximum Transmission Unit (MTU) in simple words is the maximum IP packet size in bytes, that can be transmitted over the underlying network. While most of time this never has to be changed from the default values there are instances when you need to alter the MTU sizes to fix certain network performance issues.

Windows XP

Download the file, unzip and double click on acevpn-mtu-fix-windows-xp.reg to run. Hit “OK” to install when prompted. Reboot your PC and try again.

Windows 7 and Vista

In acevpn-premium-udp-faster or the connection profile you use, add the following and restart openvpn.

mssfix 1300

Alternative Step. Do only if the above step does not fix the issue

Connect to acevpn

Go to Programs > Accessories > Command Prompt >
Right Click on Command Prompt > Select “Run as administrator”

You can only change the MTU value with elevated privileges. Even if you have administrator privileges you will still need to run cmd as Administrator. If you miss out this step then you will get the error “The requested operation requires elevation”.

Listing all the interfaces

Run the following command

netsh interface ipv4 show subinterfaces


Choosing the correct interface

In the command prompt run the following command.

ipconfig /all


Make a note of the exact name of the network device used by the VPN. In the above example they are the words directly after adapter ie “Local Area Connection 2″.

Run the following command to set the MTU value. Replace “Network Connection” with the name of the network device you made a note of above. If mtu 1400 does not help try 1300.

netsh interface ipv4 set subinterface "Network Connection" mtu=1400 store=persistent

In our example we would be running

netsh interface ipv4 set subinterface "Local Area Connection 2" mtu=1400 store=persistent


You should get the response “OK”. If you get “File not found” the interface is either not available or you have a typo. Also you need to login to the VPN first before you try this.

Restart your PC. The changes will not be set unless you reboot your PC.