Controllers are the actual VNS3 instances that are included in a topology and are allowed to be peered in a mesh.

VNS3 controller instances peer by making secure VPN connections to each other in a true mesh to share routing information and to pass traffic between each other based on the connections each Controller is servicing. Multi-Controller topologies allow for Overlay Network traffic load balancing and failover.

In this example setup diagram (see image) if one VNS3 Controller instance fails, all the Overlay Network client servers automatically connect to another VNS3 Controller in the topology.