Windows-set MTU vs Router-set MTU

Discussion in 'Network questions and troubleshooting' started by EnthusiastX, Jul 26, 2020.

  1. EnthusiastX

    EnthusiastX Member

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    EVGA RTX 3090
    I use a VPN that utilizes WireGuard protocol, which has maximum MTU size of 1420. To make sure, I tested that with "ping -f -l" commands. 1392 was the highest MTU that did not require fragmentation. If you add 28 to that like you're supposed to, you get WireGuard's MTU of 1420. I used NetSH commands to set my LAN to use MTU of 1420.

    The problem is that setting MTU of 1420 in my router results in me not being able to visit websites. Other devices work fine 1420 is set to MTU in router, but my PC doesn't...

    What I want to do know is whether Windows MTU settings override router MTU settings and whether either of them matter, given how VPN always shows MTU of 1500 at the end of the tunnel.
  2. 386SX

    386SX Master Guru

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    AMD Vega64 RedDevil
    Windows settings override your local settings. Your router has its own MTU, as every network device has.
    If you send a packet through another device, it depends on this device's MTU and the size of your MTU to determine if the packet gets fragmented.
    If your MTU is bigger then YES.
    If the other device's MTU is bigger it won't get fragmented.
    Fragmented means it gets splitted into multiple parts for the MTU to match again and then it gets forwarded to the destination you want. For example if you set MTU to 1500 and send a packet with 3000 bytes raw data it will be converted to 3 packets. 1472+1472+56 (+1416 bytes as filler to reach 1472 bytes in the third packet).
    That takes time and performance, so you don't want that to happen.

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