PPPoE over vlan RFC4638 MTU issue

[eymey]

I wanted to reply to https://forum.opnsense.org/index.php?topic=11733.0 but, probably since it is about the 20.1 series, I can not reply there any more. So I am opening this new topic (and I think it is also related to https://forum.opnsense.org/index.php?topic=16159.0).

I am encountering exactly the same problem.

Situation:
* ISP who needs a PPPoE connection over a VLAN (VLAN 6)
* IGB driver (Intel Gigabit NIC, connected with 1 Gbps to a FttH mediaconverter)
* Want to enable RFC4638
* Connection working completely, so in the GUI, the WAN interface is configured based on the PPPoE connection

* GUI -> Interfaces -> WAN:
  MTU is set to 1508 ( PPPoE caldulated MTU is 1500 )

* GUI -> Interfaces - > Point-To-Point -> pppoe0 -> advanced settings:
  MTU under the section " Link Parameters ( igb0_vlan6 )"  is set to 1508

By default, in the GUI, ther now is no configuration possible for the parent interface (igb0), so not possible to set the MTU to 1508 there. As a consequence, after every reboot, the internet connection is not working because igb0 as well as igb0_vlan6 have their MTU set to 1500.

I assume there is an attempt somwhere to put the MTU of igb0_vlan6 to 1508, as I have configured that in the advanced settings for the PPPoE device, but that attempt will surely have failed because the "igb0"  parent device still has it's MTU at 1500 (when I try to use ifconfig to put the VLAN interface at 1508, it fails if I did not change igb0).

So it looks like some changes in the OPNSense / GUI behavior have to be implemented to make this work:
* Either, when some VLAN interface has a MTU set, the parent interface should be configured automatically to the highest of configured MTU's for any ' child interface' .

* Or, please implement a GUI screen where MTU's can be configured for interfaces that are a parent of e.g. a  VLAN interface with PPPoE, but to not need further specific configuration themselves.

edit: The workaround given by the original TS, to create a sort of 'dummy interface'  for igb0, with no IP configuration, just the MTU of 1508, seems to work. But, in my opinion, give unnecessary clutter, e.g. in the firewall sections.