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In that TP-Link article its mentioned that http://www.tp-link.us/FAQ-190.html "Due to additional complications, VPNs require a different type of MTU test" I have adsl2+ model router. The MTU size for my network is 1432+28 = 1460. I am using ADSL model and config mode is PPPOE

Now I connected to one of the VPN servers and then ran the command (ping www.google.com -f -l 1472) to check the MTU. The MTU size came out to be different while connected to the VPN 1472 + 28 = 1500 as opposed to 1432+28 = 1460 when I am not connected to the VPN.

So what MTU should I put in my modem router settings 1500 or 1460 or some other value, someone told me to use 1492.

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I think there is a few concepts there:

  • Ethernet MTU is 1500 bytes.
  • IP overhead is 20 bytes, and TCP overhead is another 20 bytes, which defines the MSS (Maximum Segment Size), so 1460 bytes. In other words, the MSS is the "payload" each packet may carry.
  • The 1492 bytes long MTU is usually used on PPPoE connections (so, you would use this one in your case), which is 1500 minus 8 bytes that this protocol chomps to the original MTU size.
  • Always keep in mind that MTU is the maximum transfer unit, which doesn't mean that this will actually be the final packet size, but rather the maximum size it can handle. The client will need to negotiate with the server and agree on an actual packet size they will use taking into account the MTU of both.

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