tap(4) tells me "the tap driver can no longer be used as a bridge(4) endpoint because it supports a link state based on if it has been opened or not. Use the vether(4) driver instead as it's been explicitly designed for this purpose."
It's been over ten years since I did all this with Xen. At that time it was straightforward to create a tap interface and attach it to a bridge for a domU How are vether interfaces created and connected to a bridge? Ideally I would like domUs to connect to the internet through br0 (with the bridge attached to either the external interface or a separate vether interface), and to each other through a second bridge -- br1. As I say, it's been a long time since I did this, so my memory on all this is vague. Gerard Lally