Best MTU for L2TPv3 mode with kernel offload? about fastd HOT 1 OPEN

I believe there are some issues in the setup you linked (and both links you posted are the same?):

• An MTU describes the maximum packet size including the inner IP header, so it should not be subtracted
• A VLAN header should only be included if you intend to run VLANs over fastd

Without VLAN, this results in a total overhead of 50 bytes for L2TP tunnels over IPv4; 54 bytes with VLAN. The overhead without VLAN is also described in the docs: https://fastd.readthedocs.io/en/stable/manual/mtu.html

I assume you intend to run batman(-adv?) inside fastd. As this is another inner layers inside the tunnel interface, it doesn't affect the MTU of the fastd interface - instead, it would have to be the bat0 MTU. bat0 is often bridged with other interfaces though, so reducing bat0's MTU is usually impractical. More commonly, bat0 is left on 1500, and MSS clamping is applied instead to avoid fragmentation by batman-adv.

from fastd.