A third-party application delivery controller (ADC) is necessary in a non-layer 2 adjacent configuration, where one or more nodes in the cluster are not in the same subnet as the others. In this schema, at least one node is required to traverse through a network router in order to connect to the other nodes in the cluster.
In a deployment with a third-party ADC, the cluster does not directly respond to or manage the VIP. The ADC will respond to the VIP address and direct those connections to the active server in the cluster. The cluster provides an indicator that allows the ADC to determine which particular server in the cluster is the active node.
During a failover event the standby node with the highest priority assumes the active role. The indicator for that node will reflect the new status, allowing the ADC to begin to direct connections to it. In this schema, the VIP remains with the ADC while the cluster nodes change standby and active roles.
