OpenWRT: RFC4638 PPPoE small jumbograms and DSA

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.

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Volgens mij moest ik bij de Turris Omnia ook de ethernet interface (met de SFP) expliciet up 1508 zetten.

PPPoE overhead is 8 bytes, niet 4 bytes. Zie RFC2516 sectie 7.