For those of you affected by poor DSL performance since Thursday morning, we have discovered a workaround until the underlying network problem is resolved.  The solution is to set your network MTU to 1250.

If you have a router, change the MTU on the Internet interface, which should fix the problem for all computers on your network at once.  Consult your router’s documentation for details.

Note, the following instructions are of advanced nature. We provide these instructions on an ‘as provided’ basis. If you do not follow the instructions properly, or use them on a different type of computer than described, it may make the problem worse.

Windows 7 or Windows Vista computers do the following:

1. Click on Start. Choose Accessories.
2. Right click on “Command Prompt”. Choose Run as Administrator.
3. You should get a screen reading “Administrator: Command Prompt”.
4. Type the following:
   netsh interface ipv4 set subinterface “Local Area Connection” mtu=1250 store=persistent
   Or if you use a Wireless connection type:
   netsh interface ipv4 set subinterface “Wireless Network Connection” mtu=1250 store=persistent
5. The command should return “Ok.”.  If it doesn’t, you may not be running the command prompt as Administrator.
6. Reboot your computer.  Or disable, then enable your Local Area Connection.

The above instructions are a summarized version of this article by Richard Slater (thanks!).

Macintosh users can easily set their MTU in their System Preferences:

1. Open System Preferences. Choose Network.
2. Select “Built-In Ethernet” or “Air-Port” (if you use wireless) from the “Show” menu.
3. Go to “Advanced Settings” (Leopard or newer), or “Etherenet” (Tiger and earlier).
4. Change MTU to Custom, and enter 1250.
5. Click on “Apply Changes”, and close System Preferences.
   If the settings are greyed out, click the padlock icon on the bottom left to unlock the settings.

Linux systems:

1. Type the following in a terminal window:
   sudo ifconfig eth0 mtu 1250

Verifying your MTU:

1. Open a web browser. Go to:
   http://speedguide.net/analyzer.php
2. The second line of the report will show your MTU = 1250.  It will also display the following:
   MTU = 1250
   MTU is not fully optimized for broadband. Consider increasing your MTU to 1500 for better throughput. If you are using a router, it could be limiting your MTU regardless of Registry settings.

To return your system to normal, repeat the instructions above with the value of 1500.

This entry was posted on Friday, May 20th, 2011 at 12:09 pm and is filed under Support. Both comments and pings are currently closed.