Instructions
Open your web browser and navigate to your default IP address. This will vary depending on the type of router you have, but is generally 192.168.1.1 or something similar. Contact your Internet service provider or router manufacturer if you are unsure.
Enter the username and password required to access your router's settings. If you have set up your own, enter that. If you have not set up a username or password, you may be required to enter a default username or password. Refer to the documentation that came with your router for what information to enter when prompted.
Locate the MTU settings page. This may be located in the "Advanced" tab or "WAN" tab of the settings page or somewhere else depending on your router. Refer to your router's user guide for information on where the MTU setting is located.
Change the minimum MTU number to 1364 and the maximum to 1492. Save the settings when you are finished.
Reset your Internet connection. Turn off your computer and Xbox, power off or unplug your router and power off your modem. After a few moments, power on the modem, power on the router and try connecting to Xbox Live once your network initializes.