Setting PPPoE MTU in web interface to 1500 did not work

For RFC4638 / ppp-max-payload with an MTU of 1500 to work, the underlying Ethernet interface needs to have its MTU set to 1508 before starting pppd. Otherwise pppd will use min(requested MTU, Ethernet MTU - 8).

In cases where the PPPoE connection is made over a tagged vlan interface, both the virtual interface (vlan6 in my case) and the actual ethernet interface (eth0) needs to have its MTU set to 1508.