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About BEST and MAX MTU with PPPOE + PPTP in the router.

I have Has been setup WAN to PPPOE and MTU is 1492, because 1464 is maxed on my ISP. and I have connected PPTP on the same router, and MTU is 1942 max is 1372, so need I change WAN MTU to 1400?

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The M in MTU stands for maximum (transmission unit), so there's no "max MTU".

When determining the MTU in a tunnel you need to know the expected overheads from the wrapping protocols:

  • PPPoE: 8 bytes
  • IPv4: 20 bytes
  • IPv6: 40 bytes
  • TCP: 20 bytes
  • UDP: 8 bytes
  • PPTP (GRE): 40 bytes

The MTU outside the tunnel minus the summed overhead is the MTU inside the tunnel (unless the tunnel streams packets end-to-end, then the MTU depends on the tunnel parameters).

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  • About BEST and MAX MTU with PPPOE + PPTP in the router, need I change WAN MTU to best?
    – user46149
    Apr 5 '18 at 4:56
  • Please check and correct your question which is not entirely clear and there seem to be typos.
    – Zac67
    Apr 5 '18 at 13:27
  • I want to setup best and max MTU to my router WAN port settings. I use PPPoE (ISP) 1st and connect a PPTP VPN.
    – user46149
    Apr 5 '18 at 16:29
  • That totals to an overhead of 68 bytes (PPPoE+IP+GRE) and should be able to work with a 1432 byte MTU inside the tunnel for IPv4 outside or 1412 byte for IPv6 outside.
    – Zac67
    Apr 5 '18 at 16:59
  • I need setup 1412 to 1412 router WAN (PPPOE settings)?
    – user46149
    Apr 6 '18 at 9:28

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