I been reading xPON technology especially GPON standards as
Malaysia ISP using it as part of my project GPON Stick ONU SFP

I notice that GPON standards has fixed L2 MTU which is 2000 bytes

I login into my Mikrotik and set MTU to 1508 bytes (Baby Jumbo Frames) on sfp1 and vlan1.621, also set PPPoE MTU to 1500 bytes:
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Initiate PPPoE Connection... It works without MRRU (Active Links: 1)!
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I just to make sure run a ping MTU test:
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Ping MTU test via Windows (1500 - 28):
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For me, Maxis Fibre support RFC4638 via imse01.mc_rsh-re0 [F0:1C:2D:22:87:C3]

How about other Fibre ISP?
Can anyone try PPPoE MTU 1500? See which ISP support RFC4638 (you may need router that support)

Using pure non modified 1500 MTU from LAN to the Server.

Since PPPoE reduce MTU, some webserver have strict firewall only 1500 MTU can be access.

Making baby Jumbo (MTU 1508) from ROUTER to ISP.

So, can open many website, server, etc...

I also store "some stuff" on remote server only MTU 1500 connection can access, block TM or even MCMC to look my remote server, surely they use MTU 1492