L2 over L3 scenario
L2TPv3 works fairly well (better than I expected). I did some lab testing on it last year before deploying it for a customer where I had no other choice. I’d always pushed it to the side in favour of MPLS pseudowires and prior to last year I had only used it where I had to and the requirement was minimal (minimal traffic volume).
Last year I had a customer than needed multiple 100Mbps L2 links for backing up data using a layer 2 application/service. Any typical Cisco
ISRG2 like will process L2TPv3 in hardware so I dropped some 1941’s in and it “just worked”. They max out their 100Mbps fibres ever night with L2TPv3 tunnels between sites.
If you use port based L2TPv3 tunnels on the built in interfaces they do support the forwarding of layer 2 control frames too, so spanning-tree, CDP, LAG/LACP, UDLD etc also work. I’m now about to deliver this again possibly with 2921s for higher throughput for another customer and this customer will be runing STP or LACP, we’ve tested both in the lab and the different failure scenarios and it all “just works”. They can just add additional VLANs on their switch trunks facing the 2921s and extend extra VLANs between sites without input from me, port based if ignorant and just transport whatever it recieves.
My only gripe is no layer 2 QoS.
I did some testing with bridges too when spanning tree is present, as I don’t normally use them in such a scenario. You may need to use bridges in a scenario where you are extending layer 2 through something like an ISR-G2, IRB does interfere so it’s no advised: