Ok, support response is:
“RouterOS simply allocates 20 bytes headers. You can manually set the MTU and MRU values for the interface if other values are suitable. There is no need to increase the MTU on the ethernet interface.”
So they won’t do anything about it.
“What about the users that leave it to auto?” “- they won’t feel the difference”. (They won’t feel it maybe because in MikroTiks eyes people that like to leave stuff to auto aren’t “professionals”)? BUT I’d like my damn client to negotiate whatever it can support to whatever the ISP is giving me without undocumented 20 bytes of extra room for ghosts.
Rubbish, for short. Meh.
Where do we go from here? Either set 1520 for Ethernet like you’ve said above and leave MTU/MRU to auto so they can negotiate whatever they want without the 20 bytes allocated by RouterOS OR set MTU/MRU manually to whatever the ISP supports and leave the Ethernet MTU alone.
Quick search on the forum .. ain’t the first talk about this:
http://forum.mikrotik.com/t/pppoe-mtu-issue/117983/1
Might be others.
PS: I wonder if other vendors treat PPPoE like this too.
PS2: In a country where one of the largest ISPs is using PPPoE this sucks. For IPv6 it sucks even more. Not every user of MikroTik likes to play with knobs they don’t know what they do exactly.
Good thing this ISP doesn’t use MikroTiks for their networking stuff, 12 bytes from me, 12 bytes from x, 12 bytes from y add up on those Routers (in the more fragments on the routers → more packets to send kind of way).
The ISP that IS using MikroTiks is giving me 1480 MTU and 1500 MRU. Guess they like to keep stuff to auto/defaults too.
But hey! who cares! it’s only 12 bytes.