Let’s not mix apples with oranges.
Layer 3 MTU is one thing, Layer 2 (L2MTU) is another.
Ethernet MTU refers to how much the IP (and others) data is.
VLAN MTU means how much big can be the data that internal ethertype can carry.
If you have one ethernet with MTU of 1500, you notice that a VLAN with MTU 1500 still work and is not limited by 1496???
Is because VLAN “use” L2MTU of the ethernet…
PPPoE MTU means how big the IP payload can be carried.
If PPPoE add 8 bytes, and the PPPoE client is by VLAN, you must not increase ethernet MTU size to 1508, but leave ethernet MTU to 1500 and increase L2MTU to at least 1512,
because VLAN use L2MTU of the ethernet.
It is unlikely that an ethernet L2MTU will be less than 1512 needed (1500 IP + 8 PPPoE + 4 VLAN)
Setting larger MTU or L2MTU is absolutely useless, in fact it makes everything worse.
Their use is recommended only to exchange large amounts of data between servers connected to the same network, not via the internet.
Rarely, if not very expensively, does an ISP give the possibility of using values greater than 1500, because currently it is the basis of the entire internet.
In fact, those who use jumbo frames then have problems communicating externally because the router has to fragment (when it can)
everything that is not TCP (and currently there is a lot of UDP…) and change the MSS in the TCP connections…
http://forum.mikrotik.com/t/pppoe-mtu-always-defaults-to-1480-instead-of-1492/181991/81