When creating a local network in Neutron, it is not possible to specify a VLAN ID or even a physical interface. The Neutron Linux bridge agent will create a bridge and connect only the tap interface of the instance to the bridge. Instances in the same local network on the same node will be connected to the same bridge and are free to communicate with one another. Because the host does not have a physical or virtual VLAN interface connected to the bridge, traffic between instances is limited to the host on which the instances reside. Traffic between instances in the same local network that reside on different hosts will be unable to communicate with one another.

Figure 4.5 demonstrates the lack of physical or virtual VLAN interfaces connected to the bridge:

In Figure 4.5, two local networks have been created along with their respective bridges, brqZZZZ and brqNNNN. Instances connected to the same bridge can communicate with one another, but nothing else outside of the bridge. There is no mechanism to permit traffic between instances on different bridges or hosts when using local networks.

Some application architectures may require multiple instances be deployed on the same host without the need for cross-host communication. A local network might make sense in this scenario and can be used to avoid the consumption of precious VLAN IDs or VXLAN overhead.