Onderstaande tekst zojuist gepost in het OpenWRT forum. Maar mogelijk dat er hier ook mensen zijn die iets hebben aan deze info.
Here’s a comment that is hopefully useful to some: my ISP (Freedom internet) delivers PPPoE via VLAN6. PPPoE adds a 4-byte header; VLAN adds a 4-byte header; with a standard 1500-byte MTU that leaves 1492 bytes and hence, by default, the MTU on a connection like this is not 1500, but 1492 bytes.
With my old Archer C7 router, I would just set the MTU of the WAN PPPoE connection to 1508 (default is 1500) and OpenWRT will happily negotiate RFC4638 resulting in a 1500-byte connection.
However, I recently changed to Ubiquity ER-X (which is recommended, very cheap and will do 1gbit/s traffic) which brings DSA. And I had trouble making RFC4638 work again.
The trick is to manually set a MTU of 1508 on the ethernet port to the provider (VDSL, or AON or GPON fiber), in my case eth0 in the Interfaces->Devices menu. OpenWRT will automatically configure the VLAN port eth0.6 to a MTU of 1508 as well.
Then you also need to configure a MTU of 1500 on the WAN interface (Interfaces->Interfaces->wan->Override MTU). OpenWRT will disconnect, reconnect and handshake RFC4638 and all is well.I did not find this in the DSA documentation and initially, the MTU of the WAN port was automatically reset to 1492. But this is how it works for me and I thought I should report out. Hope this is useful to someone. Oh, and all of this on 23.05.0, didn’t try other versions.