Description.

The ML2/OVN mechanism driver supports the allowed address pair extension. It is recommended to create a port with the IP address to be shared with the "main" one. This is a way to "reserve" the IP address. This port will be marked as virtual. E.g.:
$ openstack port create --network private port_vm
$ openstack port create --network private --fixed-ip subnet=sprivate,ip-address=10.0.0.10/26 port_virtual
$ openstack port set --allowed-address 10.0.0.10 port_vm

At this port, ML2/OVN will mark the Logical_Switch_Port port_virtual as "virtual".

However, if a CIDR is used, no port (Logical_Switch_Port) inside this CIDR will be marked as "virtual". E.g.:
$ openstack port create --network private port_vm
$ openstack port create --network private --fixed-ip subnet=sprivate,ip-address=10.0.0.10/26 port_virtual
$ openstack port set --allowed-address 10.0.0.0/26 port_vm

In this case, port_virtual is NOT set as "virtual".

Goal.

Document this behaviour.

U/S bug: https://bugs.launchpad.net/neutron/+bug/2132977