When creating a cluster for a multiple site configuration with PowerHA® SystemMirror 7.1.2 Standard Edition you can configure a cluster in one of two modes.
When you use a stretched cluster in your environment, a single Cluster Aware AIX® (CAA) cluster is deployed across the sites in the cluster. If internet protocol (IP) communication fails and your environment is using stretched clusters, all nodes in the cluster can use the disk heartbeat function to communicate and keep your environment functioning.
CAA uses linked clusters to support multiple sites that are geographically far apart, usually in different cities. Linked clusters link the individual CAA clusters to these sites. PowerHA SystemMirror uses the links between the sites for heartbeat and cluster communications. These links use regular IP communication between the sites, instead of the multicast IP communication that each CAA cluster uses internally.
Using linked clusters is a useful option if you cannot use multicasting between the sites or if you cannot share a common repository disk between the sites.
For more information about cluster types for multiple site configurations, see the Stretched clusters topic and the Linked clusters in the AIX Information Center.