Networking API has a provider networks extension that you can specify provider extended attributes for a network. Tempest doesn't have test coverage for this.
provider:physical_network (Optional)
The physical network where this network object is implemented. The Networking API v2.0 does not provide a way to list available physical networks. For example, the Open vSwitch plug-in configuration file defines a symbolic name that maps to specific bridges on each Compute host.
provider:network_type (Optional)
The type of physical network that maps to this network resource. Examples are flat, vlan, vxlan, and gre.
provider:segmentation_id (Optional)
An isolated segment on the physical network. The network_type attribute defines the segmentation model. For example, if network_type is vlan, this ID is a vlan identifier. If network_type is gre, this ID is a gre key.
Assigned to myself to implement it.