The ppp daemon in FreeBSD 11.2 limits the maximum MTU to 1492 for the ppp/tun device. Changing the MTU and MRU in the ppp.conf file to values above 1492 doesn't work. This is because the limit is hard coded in
My ADSL modem (vigor 130) supports an MTU above 1500 and if I change the limit in ether.c to 1500, set my ethernet mtu to 1508 or more, then ppp will negotiate a higher MTU/MRU for the link and it will work fine (I tried 1500 byte ping probes with the DF flag set and it works)
Unfortunately, you can't just change the ether.c value because it uses the limit as the default and so my old modem (can only do 1492 or less) will not work unless I set the MTU to 1492 in the ppp.conf file. It would probably be best to leave the default at 1492 since this is the most common maximum MTU for ADSL modems, but allow it to be overridden to whatever you want in the config file.
To be clear, this issue only applies to PPPoE,
You can use mpd5 port or package that can act as PPPoE client and has support for mtu over 1492.