So if the newly created interface has a lower MAC, the bridge changes its MAC address and uses that of the new interface. Now, by default bridge interfaces in Linux use, for their MAC address, the lowest MAC address among the enslaved interfaces.

It is theoretically possible to set the MAC address of this host-side tap interface, but it's quite involved and basically nobody does that (and also virt-manager has no provision to do that). (Note that we're talking of the host-side tap interface, not the guest-side virtual interface, whose MAC address is configurable in the guest's configuration file or on the command line). Minor but important detail, this tap interface has a more or less random MAC address.

This interface has a name like vnet5, for example. What happens here is that, when a new guest is started, a tap interface is created and enslaved to the bridge (the tap interface is usually connected to the guest's own ethernet interface). Initially one might think of something related to STP, but it turns out that it is disabled (and even then, ports appearing or disappearing should not affect existing ports). The problem is, the host seems to randomly suffer some loss of connectivity (from a few to 30-40 seconds) when some guest is started or stopped. The host has a bridge interface br0 that starts out containing only eth0, and other interfaces are dynamically added and removed from the bridge as guests are started and stopped. Scenario: KVM virtualization host running several bridged guests.

Linux get network mac address