OpenStack Neutron currently has a feature referred to as "provider
networks". This is used as a way to define existing physical networks
that you would like to integrate into your environment.

In the simplest case, it can be used in environments where they have no
interest in tenant networks. Instead, they want all VMs hooked up
directly to a pre-defined network in their environment. This use case
is actually popular for private OpenStack deployments.

Neutron's current OVS agent that runs on network nodes and hypervisors
has this configuration entry:

bridge_mappings = physnet1:br-eth1,physnet2:br-eth2[...]

This is used to name your physical networks and the bridge used to
access that physical network from the local node.

Defining a provider network via the Neutron API via the neutron
command looks like this:

$ neutron net-create physnet1 --shared \
> --provider:physical_network external \
> --provider:network_type flat

A provider network can also be defined with a VLAN ID:

$ neutron net-create physnet1-101 --shared \
> --provider:physical_network external \
> --provider:network_type vlan \
> --provider:segmentation_id 101