Microsoft KB Archive/314100

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How to Establish the Allowable MTU Size When You Are Using Internet Connection Sharing on PPPoE


The information in this article applies to:

  • Microsoft Windows XP Home Edition
  • Microsoft Windows XP Professional

For a Microsoft Windows 2000 version of this article, see Q259783.


This article describes how to establish the maximum transmission unit (MTU) size for using Internet Connection Sharing (ICS) if your outbound connection uses Point-to-Point Protocol over Ethernet (PPPoE). If the setting for MTU size is too high, clients that are using the ICS connection may not be able to browse some Web sites or send messages that contain attachments.


PPPoE is a method for establishing Point-to-Point Protocol (PPP) connections through Ethernet network adapters. PPPoE requires that, on all client computers, you lower the setting for maximum transmission unit (MTU) size to a value no higher than 1,492. (The default is 1,500.) In some cases, a value lower than 1,492 may be necessary.

To identify the correct MTU size, follow these steps:

  1. Start a command prompt, and then determine the default gateway of the ICS server by running the Ipconfig utility.
  2. At a command prompt on a client, type the following, and then press ENTER:

    ping -f -l MTU_size default_gateway_IP_address

    Start with an MTU size of 1,454.

  3. If an error message indicates that the message must be fragmented, decrease the MTU size, and then run the command again. Repeat this process until the ping command succeeds.

After you determine the largest MTU size that is supported, manually set the MTU.

For additional information about manually setting the MTU, click the article number below to view the article in the Microsoft Knowledge Base:

Q314053 TCP/IP and NBT Configuration Parameters for Windows XP

PPPoE is described in RFC 2516 and is currently being used by digital subscriber line (DSL) service providers to maintain the familiar "dial-up" experience for users who connect to the Internet through a DSL modem. Additional query words:

Keywords : kbnetwork kbStack
Issue type : kbinfo
Technology : kbWinXPHome kbWinXPPro kbWinXPProSearch kbWinXPHomeSearch kbWinXPSearch

Last Reviewed: January 16, 2002