openstack:

  networks:
  - internal ( 192.168.200.0/24)
  - external ( 10.0.0.0/8 )
  - public ( public IPs pool)
  routers:
  - ext-router (gw: external , internal interface )
  - pub-router (gw: public , internal interface )

netplan.yaml:

  network:
      version: 2
      ethernets:
          eth0: # internal
            addresses:
            - 192.168.1.10/24
            gateway4: 192.168.1.1
            nameservers:
              addresses:
              - 192.168.1.1
          eth1: # public ( neutron_external_network )
              routes:
              - to: 10.0.0.0/8
                via: <ext-router-fixed-ip>
                metric: 200
              - to: 0.0.0.0/0
                via: <pub-router-fixed-ip>
                metric: 300
          eth2: # external ( neutron_external_network )
              routes:
              - to: 10.0.0.0/8
                via: <ext-router-fixed-ip>
                metric: 200
              - to: 0.0.0.0/0
                via: <pub-router-fixed-ip>
                metric: 300