Hello,
STP BPDUs should be passed over the tunnel by default, without needing any extra configuration from Teltonika device side. All that needs to be done is the L2TP tunnel should be up and running. Just to confirm this - I've tested it with two routers connected and one host and then performed another test using a managed Unifi switch with RSTP enabled - in both cases the packets showed up on both routers (using tcpdump) as well as the end-machine (using wireshark). During my testing the topologies I used were the following:
- RUTX12 ~~ <L2TP/IPsec tunnel over WAN> ~~ RUT955 --> PC (wireshark)
- PC1 (wireshark) <-- RUTX12 ~~ <L2TP/IPsec tunnel over WAN> ~~ RUT955 --> Unifi switch --> PC2 (wireshark)
In both cases the BPDUs arrived to my end devices either directly or via tunnel. This was tested both with STP and RSTP.
If this doesn't work for you for some reason then please let me know the models of your devices (including any switches), VPN configuration as well as troubleshoot files from any Teltonika devices.
Best regards,
Tomas.