Configure Max Transfer Unit (MTU) size for the TCPIP stack

The default Max Transfer Unit (MTU) size for ethernet is 1500 bytes, if using a network with a different MTU size, then one should make sure to configure MTU in Windows to avoid packet fragmentation and loss of VPN connections.

If having several adapters in the registry, one can find the wanted Adapter, by making changes to the TCPIP configuration for the Adapter in Network Properties. Example set an odd DNS-Address for TCPIP protocol bound to the adapter, and then look at the TCPIP setting for each adapter to find the odd DNS-Address.

To configure MTU in WinNT4:

  • Adapter MTU (Go to the Services-key and do a search for “TCPIP” to find the different adapters using TCPIP)

    [HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \{Adapter-Name} \Parameters \Tcpip]
    MTU = 1500

  • Point-To-Point Protocol (PPP) MTU (WinNT4 SP4+)

    [HKEY_LOCAL_MACHINE \System \CurrentControlSet\Services \NdisWan \Parameters]
    IPMTU = 1500

    More Info MS KB183229

  • Virtual Private Network (VPN) Tunnel MTU (WinNT4 SP4+)

    [HKEY_LOCAL_MACHINE \System \CurrentControlSet\Services \NdisWan \Parameters]
    TunnelMTU= 1400

    More Info MS KB183229

Related Configure Max Transfer Unit (MTU) in Windows 2000 / XP