Using bridges under NixOS
From NixOS Wiki
Using bridges under NixOS
If you want to use macvtap bridges in KVM, LXC, Xen or $hypervisor on NixOS you have to tell your dhcp server to not configure that interface with an ip. Yes exactly the dhcp server on NixOS configures every interface as long it has been told to not to.
If you are running dhcpcd then you have to define
networking.dhcpcd.denyInterfaces = [ "macvtap0@*" ]
for example.
What is the error you get when layer 2 bridges have IPs? stackoverflow issue