MTU Size issue

[greyfreak19]

so i have noticed that if i use the Custom MTU Size checkbox and use 1492 for my PPPOE DSL connection on my pc it shows max 1484 instead of 1492 so i set it to 1484 in my router and the max mtu size decreased from what i have set in the router i then changed it to 1400 thinking my isp was the issue but it happened again the max should have been 1400 instead it was 1392 then i set 1492 back into the router nothing changed max was still 1392 i unchecked the custom MTU tick box it was still 1392 restarting didnt help i then had to reflash the firmware and didnt touch the custom mtu tick box and the max was back to 1492 i checked on DD-WRT and it wasn't an ISP issue tried 1.8.1 and 1.9.0 both have the same results i tried this many times the same thing happened

[Lantis]

Notice it's subtracting 8 each time? This is for the PPP packet headers. Now I'm not great with MTU, but I think the PPPoE interface needs to be 1500, and the Ethernet interface 1492. Therefore we add the PPP headers on the way out and get to 1500.

If you set it to 1492, the Ethernet gets set to 1492-8=1486

[greyfreak19]

so gargoyle automatically compensates for PPP packet headers while on most other firmware if i set 1492 i get 1492 so 1500in gargoyle should equal to 1492 if i use PPPoE well i didn't now that

[Lantis]

It's not gargoyle it's openwrt and the pppdaemon.

I didn't know until you asked the question. I then looked into it and provided my thoughts. I could be wrong though.

[greyfreak19]

Well atleast i know i don't need to mess with the custom MTU option