PowerHA® SystemMirror
uses multicast addresses for node failure detection,
which indicates to the cluster manager when a fallover operation might
be needed. A multicast address is also known as a class D address.
PowerHA SystemMirror uses a cluster health management layer within the AIX® operating system. This layer is called Cluster Aware (CAA). CAA uses AIX kernel-level code to exchange heartbeats over a network. This heartbeat exchange uses Fibre Channel adapters and uses disk-based messaging through the central repository for the cluster. CAA uses a multicast IP-based network communication mechanism to communicate between different nodes in a cluster.
If you configure a linked cluster with
multiple sites, you must specify one multicast address per site. A linked cluster contains
nodes from sites at different geographical locations. Linked clusters
do not require sites to share a repository disk or to support multicast
communication.
For all other cluster types (non-site and stretched),
only a single multicast address is needed for the entire cluster. A stretched cluster contains
nodes from sites at the same geographical locations. Stretched clusters
share at least one repository disk between sites.
If you do not provide the required multicast address,
CAA creates one for you.
For more information about multicast addresses, see the Multicast addresses topic in the AIX Information Center.