Mac OS X 10.4 or later: How to change the MTU for troubleshooting purposes

If your broadband Internet connection (cable or DSL) is slower than expected, your Internet Service Provider (ISP) may suggest that you change the maximum transmission unit (MTU) as a troubleshooting step.

Important: Changing the MTU is recommended only when done under the advice of your ISP or an experienced troubleshooter. Improper configuration can result in a loss of connectivity.

Steps for Ethernet in Mac OS X 10.5.x

  1. From the Apple menu, choose System Preferences.
  2. Click Network.
  3. In the left sidebar, select "Built-in Ethernet 1".
  4. If you see a lock icon in the bottom left corner, click the lock icon and enter your administrator password.
  5. Click the "Advanced..." button.
  6. Click the Ethernet tab.
  7. From the Configure: pop-up menu, choose "Manually".
  8. From the MTU pop-up menu, choose "Custom".
  9. Enter your desired setting (72 to 9000). Some ISPs may have a specific suggested MTU value. If you're changing it without a specific recommendation, make incremental changes (such as 1500 to 1400 to 1300, and so forth).
  10. Click OK, then click Apply to save your changes.
     

Important: Changes made to your Ethernet MTU settings will be persistent, meaning the new values will still be used after you restart your computer.

Steps for AirPort in Mac OS X 10.5.x

  1. From the Apple menu, choose System Preferences.
  2. Click Network.
  3. From the Show pop-up menu, choose AirPort.
  4. Click the Ethernet tab.
  5. From the Configure pop-up menu, choose Manually.
  6. From the MTU pop-up menu, choose Custom.
  7. In the field to the right of Custom, enter your new setting. Some ISPs may have a specific suggested MTU value. If you're changing it without a specific recommendation, make incremental changes (such as 1500 to 1400 to 1300, and so forth).

Important: Changes made to your Ethernet MTU settings will be persistent, meaning the new values will still be used after you restart your computer.

For Mac OS X 10.4.x

Note: For these steps below (Mac OS X 10.4.x), you must be logged into Mac OS X as a user who has administrator privileges. You can see which users are administrators in the Accounts pane of System Preferences.

  1. Open the Terminal, which is located in the Applications folder, inside the Utilities folder.
  2. At the command prompt, type the following command. In place of "<value>" enter the number for the MTU value you wish to set.
    $ sudo ifconfig en1 mtu 
    For example, this would set the MTU value for the AirPort interface to 1400:
     $ sudo ifconfig en1 mtu 1400
  3. You will be prompted to enter your password, and then the command will be completed. You can verify the setting of the MTU value with this command:
    $ ifconfig en1
    This will return all current information about your "en1" (AirPort) network interface as seen at the end of the second line of text in this image:

Note: The AirPort MTU setting made via Terminal is not persistent, meaning that you will need to repeat these steps each time you restart your computer.

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Last Modified: Oct 22, 2008
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  • Last Modified: Oct 22, 2008
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