In an environment consisting of a single controller and one or more compute nodes, the controller will likely handle all networking services and other OpenStack services while the compute nodes strictly provide compute resources.

The following diagram demonstrates a controller node hosting all OpenStack management and networking services where the Neutron layer 3 agent is not utilized. Two physical interfaces are used to separate management (control plane) and instance (data plane) network traffic:

The preceding diagram reflects the use of a single combined controller/network node and one or more compute nodes, with Neutron providing only layer 2 connectivity between instances and external gateway devices. An external router is needed to handle routing between network segments.

The following diagram demonstrates a controller node hosting all OpenStack management and networking services, including the Neutron L3 agent. Three physical interfaces are used to provide separate control and data planes:

The preceding diagram reflects the use of a single combined controller/network node and one or more compute nodes in a network configuration that utilizes the Neutron L3 agent. Software routers created with Neutron reside on the controller node, and handle routing between connected project networks and external provider networks.