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?
1 Answer
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?– user46149Commented Apr 5, 2018 at 4:56
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Please check and correct your question which is not entirely clear and there seem to be typos.– Zac67 ♦Commented Apr 5, 2018 at 13:27
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I want to setup best and max MTU to my router WAN port settings. I use PPPoE (ISP) 1st and connect a PPTP VPN.– user46149Commented Apr 5, 2018 at 16:29
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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 ♦Commented Apr 5, 2018 at 16:59
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I need setup 1412 to 1412 router WAN (PPPOE settings)?– user46149Commented Apr 6, 2018 at 9:28