How to Adjust the MTU in the AirPort Extreme
The maximum transmission unit (MTU) is an Internet setting that controls the data transfer rate of your high-speed Internet connection and may need to be configured when your connection is slower than usual. MTU configuration is found in the network preferences on your computer and should not be changed unless your Internet Service Provider states that adjusting it can correct connectivity issues. A conflicting MTU value in your network's preferences can result in damage to the connection including a loss of connectivity altogether. Always contact your ISP prior to making changes to the network preferences.
Click the Apple icon and click "System Preferences" from the pull-down menu. Click the Network icon in the window to access your computer's stored network preferences.
Click the Ethernet tab in the Network preferences' side menu and click the Configure pull-down menu. Click "Manually" in the menu and click the MTU pull-down menu.
Click "Custom" in the pull-down menu and enter the new value in the field next to MTU. Contact your ISP to determine the appropriate MTU value.
Click the red button in the top-left corner of the Network window to exit. Click "Apply" to apply the changes to your network. The AirPort Extreme may reset itself once the MTU value is changed. Wait until the AirPort Extreme's base station displays a solid green light before using the Internet.