How to change MTU (Windows)

Two possible scenarios:

  • You are using VPN, but some of your applications time out (MTU is too big)
  • Your OS is preconfigured for VPN by your employer, but the transfer rate can be better (MTU is too small)

In both cases the MTU needs to be adjusted:

  1. Start regedit
  2. Check all interfaces under HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces and find the one with the IP corresponding to the problem interface (e.g. the VPN tunnel interface). For example:
    IPAddress REG_MULTI_SZ
    MTU REG_DWORD 0x000005DC (1500)

    As you can see the MTU is set to 1500 what is too high for a VPN tunnel

  3. Double click on MTU
  4. Click on Decimal
  5. Enter 1400 (or 1300 to be on the safe side, or 1500 if you are not using VPN)
  6. Click OK
  7. Reconnect (if you are connected to the VPN)

Also see:

Windows 2000/XP Registry Tweaks
How to change the PPPoE MTU size in Windows XP
Change the Maximum Transmission Unit (MTU) Size (Windows NT/2000/XP)

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