neutron-linuxbridge-agent flat network incompatibility with systemd-networkd

We are running an OpenStack installation from Ubuntu's Cloud Archive, and our computer hosts have their network configured with systemd-networkd. For example, a bond and several VLANs on top, including two VLANs used for OpenStack flat networks. We are using neutron-linuxbridge-agent, i.e. no OVS or OVN.

Network interface overview:

     ┌──────┐ ┌──────┐ ┌──────┐
     │ eno2 │ │ eno3 │ │ eno4 │
     └───┬──┘ └───┬──┘ └───┬──┘
         │ │ │
     ┌───┴────────┴────────┴──┐
     │ bond0 │
     └─┬───────┬─────────┬────┘
       │ │ │
   ┌───┴──┐ ┌──┴───┐ ┌───┴────┐
   │ tnet │ │ stor │ │ public │
   └──────┘ └──────┘ └────────┘

The "public" network is a flat provider network (VLAN) that shall be used by neutron-linuxbridge-agent and nova-compute to attach to virtual machines. "tnet" is used for VXLAN overlay networks, and "stor" for Ceph. The issue is with "public" only.

The systemd-networkd configuration sets up all interfaces. We previously used netplan, but missing some needed options. It does use systemd-networkd too, so no relevant change to the problem.

Since merging https://github.com/systemd/systemd/pull/17392, systemd-network will remove the master flag from the "public" interface, that neutron-linuxbridge-agent assigned to the bridge for the network, e.g. brq88363244-5f, breaking all network on the flat network.

The fix was revered in bug #1929560, but the behavior will not be reverted in newer releases. Therefore, every reload of systemd-networkd will remove the physical network interface from neutrons bridge.

Using "bridge_mappings" with nova-compute has never been finished, according to bug #1105488. nova-compute always requires the dynamic bridge, e.g. brq88363244-5f, to be present.

@ddstreet asked for a new bug and the network configuration:

    # /etc/systemd/network/10-eno1.network:
    [..]

    # /etc/systemd/network/10-eno2.network:
    [Match]
    Name=eno2

    [Network]
    LinkLocalAddressing=no
    Bond=bond0

    # /etc/systemd/network/10-eno3.network:
    [Match]
    Name=eno3

    [Network]
    LinkLocalAddressing=no
    Bond=bond0

    # /etc/systemd/network/10-eno4.network:
    [Match]
    Name=eno4

    [Network]
    LinkLocalAddressing=no
    Bond=bond0

    # /etc/systemd/network/10-bond0.netdev:
    [NetDev]
    Name=bond0
    Kind=bond
    MTUBytes=9000

    [Bond]
    Mode=802.3ad
    LACPTransmitRate=fast
    MIIMonitorSec=100ms
    TransmitHashPolicy=layer3+4

    # /etc/systemd/network/10-bond0.network:
    [Match]
    Name=bond0

    [Link]
    MTUBytes=9000

    [Network]
    LinkLocalAddressing=no
    ConfigureWithoutCarrier=yes
    VLAN=tnet
    VLAN=stor
    VLAN=pulic

    # /etc/systemd/network/10-public.netdev:
    [NetDev]
    Name=public
    Kind=vlan
    MTUBytes=1500

    [VLAN]
    Id=82

    # /etc/systemd/network/10-public.network:
    [Match]
    Name=public

    [Link]
    MTUBytes=1500

    [Network]
    LinkLocalAddressing=no
    ConfigureWithoutCarrier=yes

    # /etc/systemd/network/10-tnet.netdev:
    [..]

    # /etc/systemd/network/10-tnet.network:
    [..]

    # /etc/systemd/network/10-stor.netdev:
    [..]

    # /etc/systemd/network/10-stor.network:
    [..]

neutron-linuxbridge-agent is configured to use "public" as a flat network, e.g.:

    # /etc/neutron/plugins/ml2/linuxbridge_agent.ini
    [linux_bridge]
    physical_interface_mappings = public:public
    [..]

When everything is started and a VM needs to be connected to the flat network, neutron-linuxbridge-agent creates a dynamic bridge for nova-compute, e.g. brq88363244-5f:

    14: public@bond0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 master brqe240c66b-da state UP
    59: brqe240c66b-da: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP

When systemd-networkd is restarted, it will see that "public" is up but has a master assigned. It will be unassigned and break the flat network:

    14: public@bond0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP
    59: brqe240c66b-da: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP

What would be the recommended why to configure networking on compute hosts with Ubuntu OpenStack and flat networks? Not using systemd-networkd or netplan at all, but only /etc/network/interfaces?

Is there any modern alternative?