Bridge L2TP interface to an ethernet port

I am trying to create a L2 bridge between an ethernet port and an L2TP tunnel so I can transparently transmit tagged and untagged L2 traffic on top of a L3 connection.

The L2TP tunnel is up and working (on top of PPP) but I cant add the L2TP interface to a bridge. What am I doing wrong? With Mikrotik I was able to add the L2TP interface to a bridge, the Openwrt/LEDE should be the client side.

Any and all help is appreciated.

Thanks!

Seeing your /etc/config/network stanzas would help, perhaps with the output of brctl show.

I bridge several gretap VLANs over Ethernet interfaces. I would imagine L2TP would be similar.

Be aware that PMTUD will often fail with L2 bridging (no clear recipient for the fragmentation-required ICMP message). This is an underlying protocol issue, not an OpenWRT-implementation issue.

That is the problem: the L2TP link is up, it can pass data, but when I try to run brctl addif and try to add the l2tp connection to the brdige, it fails with invalid argument.

Now that you mentioned gretap, maybe I am missing some dependencies? I already switched to the full bridge implementation instead of busybox, but I wonder if there is something else missing? What kind of packages/kernel modules should I check?

I've not tried to add with brctrl, but with UCI and trusting netifd to "do the right thing". An example stanza is

config interface 'vlanNNN'
        option type 'bridge'
        option stp '1'
        option ifname 'eth1.NNN @gtA.NNN @gtB.NNN @gtC.NNN @gtD.NNN'
        option proto 'static'
        option ipaddr '<the bridge's IP addr, if needed>'
        option netmask '<and its netmask>'
        option delegate '0'

The gt interfaces are gretap (L2) links.