Orange / Poland / FTTH connection.... and MTU 1500

Interestingly looking at Mikrotik Forum someone complained about mini jumbo frames not working in ROS 7rc.something... and decided to read more. 

If you know - PPPoE  protocol ( sometimes called Layer 2.5 for some reason ) is a PPP over Ethernet. It's used mostly in ADSL context and in my case FTTH (Fiber To The Home) connection. I was lucky enough (asked for it) - received Huawei ONT (Optical Network Terminal) that is connected to Mikrotik RB4011. 

Seems that this Huawei ONT is supporting mini-jumbo frames. Previously I was using standard setup where MTU for ETH interface was set to 1500 but it seems that it is possible to set higher number for my hardware. Why I want bigger eth frame? PPPoE eats up 8 bytes of the frame (PPPoE header 6 bytes + 2 bytes of id), this means that your regular sized frame cant be transmitted 1:1 to your ISP (overhead was probably minimal but still...)

In this image you see WinBox/Interface/1/General. 

I've bumped MTU to 1508 on ETHERNET interface that will receive Ethernet from ONT.

If your PPPoE connection is not working, restart ONT and wait a moment.

and proof: (ping with do not fragment option set, and size 1472 (+28 for headers so it should fit the frame!)

In general full setup to make it work looks like this:

• ONT connected to ETH1 port 
• Create VLAN on ETH1 interface with VLAN ID: 35
• Create pppoe-out interface using Vlan1 interface (and fill all required fields in Dial Out)

UPDATE:

With MikroTik FW 7.7+ automatic detection of MRU is working correctly.