The easiest way I’ve found is the following
first is to add the external interface to the br-ex in openvswitch this allows for connections to come into the stack.
something like this
ovs-vsctl add-port br-ex eno2
then you need to editing the ml2_conf.ini under /var/snap/microstack/common/neutron/ something something
you may have to copy the one from the repo https://opendev.org/x/microstack/src/branch/master/snap-overlay/etc/neutron/plugins/ml2/ml2_conf.ini
[ml2]
type_drivers = flat,vlan
external_network_type = flat
[ml2_type_vlan]
flat_networks = *
Restart your service and that should give to external vlan access.
if you just wanted a flat single network will just add the port to ovs and change the network in microstack.