I see that for some reason, ppp is hardcoded a limit for MTU/MRU at max 1500 here:
https://github.com/ppp-project/ppp/blob/master/include/net/ppp_defs.h#L55

Even if I specify the MTU/MRU number higher than 1500 then it will just jump back to 1500
I know that a standard PPPoE MTU is supposed to be 1492 to add space for the overheads, I tried to raise those numbers to work with 1600 bytes and it seems to work just fine (my BRAS supported it).

My question is that why can't we make those MTU MRU dynamically fit with whatever the lower layer support and not limit it down to 1500? I feel like there's a reason for that but the .h file didn't tell me what it is... I need a larger MTU to help with the fragmentation issue if the client is using 4in6/DSLite.