After you install Access Manger Appliance, an Identity Server cluster configuration is created automatically. If you install a secondary appliance, Identity Server in that server will automatically be added to Identity Server cluster.
In the Access Manager Appliance, Identity Server is automatically configured as a service that is accelerated through Access Gateway. Access Gateway in one appliance is configured to communicate only to Identity Server in the same appliance. However, Identity Servers in a cluster can internally communicate to each other through the cluster back channel.