The controller is a key part of the software defined WAN as it provides centralized intelligence for the WAN. In a Talari SD-WAN, any appliance can be designated as the controller, and is referred to as the Network Control Node or NCN. The appliance designated as the controller is usually in a location with sufficient bandwidth and connectivity to establish a connection with each Talari appliance in the network – generally either the data center, a colocation facility or the cloud.
Because of the importance of the NCN, it is possible to run the NCN as a high availability pair so that there is an active standby in case the primary NCN fails. And a second appliance or high availability pair in a geographically separate location can be designated as a backup NCN, which will take over in case of a complete failure of the primary NCN pair. These levels of redundancy ensure that the controller function is highly available. Even with those measures, it is possible in a WAN to experience periods where the controller is not accessible. The Talari software defined WAN is designed to continue to function even when the NCN is temporarily unavailable.
Policies that control the behavior of a Talari software defined WAN are defined in a single location – not on every appliance. The NCN is responsible for centrally storing the application and traffic handling policies and pushing changes to the client appliances.