gistfile1.txt

3: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether d0:50:99:0c:35:20 brd ff:ff:ff:ff:ff:ff        
    inet 192.168.11.2/24 brd 172.22.104.255 scope global noprefixroute br0
       valid_lft forever preferred_lft forever                                                                       
    inet6 fe80::dc09:b9ff:fe5d:756c/64 scope link     
       valid_lft forever preferred_lft forever 
       
       

netplan.yaml

---
network:
  version: 2
  # if specified globally, can only realistically have that value,
  # as networkd cannot render wifi/3G.
  ethernets:
    enp4s0:
      dhcp4: false
  bridges:
    br0:
      dhcp4: false
      mtu: 1500
      gateway4: 192.168.11.254
      addresses: [192.168.11.2/24]
      interfaces: [eth0]
      parameters:
        stp: true
        forward-delay: 4
      nameservers:
          addresses: [1.1.1.1]

Vagrantfile

Vagrant.configure("2") do |config|
  config.ssh.private_key_path = ["~/.vagrant.d/insecure_private_key", "~/.ssh/id_rsa"]
  config.ssh.insert_key = false
  config.vm.provision "file", source: "~/.ssh/id_rsa.pub", destination: "~/.ssh/me.pub"
  config.vm.provision "shell", inline: "cat ~vagrant/.ssh/me.pub >> ~vagrant/.ssh/authorized_keys"
  config.vm.provision "shell", path: "bootstrap.sh"
  config.disksize.size = '60GB'

  config.vm.provider :libvirt do |domain|
    domain.nested = true
    domain.machine_virtual_size = 60
    domain.volume_cache = "unsafe"
    domain.cpu_mode = "host-passthrough"
    domain.nested = true
    domain.memory = 1024*2
    domain.cpus = 2
  end

  config.vm.define :instance1 do |instance|
    instance.vm.box = "centos/7"
    instance.vm.hostname = 'c1'
    config.vm.provider :libvirt do |domain|
      domain.memory = 1024*4
    end
    instance.vm.network :public_network,
      :dev => "br0",
      :mode => "bridge",
      :ip => "192.168.11.78",
      :type => "bridge"

    instance.vm.network :public_network,
      :dev => "br0",
      :mode => "bridge",
      :ip => "192.168.11.79",
      :type => "bridge"

  end
end