How to make a switch with VyOS running on VirtualBox to be used by other VirtualBox machines?

Hello. I have a VyOS system running on VirtualBox. This machine has two network interfaces. I’ve made a bridge with these two interfaces (like this: configure; set interfaces bridge br0; set interfaces bridge br0 member interface eth0; set interfaces bridge br0 member interface eth1;commit ) and it seems correct by seeing show bridge br0 detail output.

Next I’ve connected another VirtualBox machine to one each of these interfaces which forms the bridge (both machines run Ubuntu). I’ve done this connection by using VirtualBox’s UDP tunnel generic driver and specifying the dest, sport and dport parameters like this:

VyOS’s eth0:
dest=127.0.0.1
dport=10001
sport=10003

VyOS’s eth1:
dest=127.0.0.1
dport=10002
sport=10004

Ubuntu1:
dest=127.0.0.1
dport=10003
sport=10001

Ubuntu2:
dest=127.0.0.1
dport=10004
sport=10002

What I´d expected is Ubuntu machines (after statically assigning an IP/mask to each one from the same network) could do ping between themselves but they don’t. I get “Destination host unreachable” error.

What am I doing wrong? Or, in general, how can achieve a bridged connection between two VirtualBox VMs through a VyOS system acting as a switch?

Thanks!

P.S: If I don’t create any bridge on VyOS system but instead I configure its eth0 interface, for instance, like a “standard one” (configure; set interfaces ethernet eth0 address 10.0.0.3/8; commit), then there is connection between this interface and the Ubuntu’s machine connected to it. So I suspect “degrading” the ethernet interface to belong to a bridge is causing the issue in the udp tunnel, but I’m stuck here.

P.S2: I don’t want to use GNS3 or the like. I want to do all the connections “handcrafted”

I got it! Ubuntu machines were clones, so their MAC address was identical. I changed it in one of them and voilà, ping has begun to work!