Hi Sean,
Cisco Unified CallManager Redundancy Groups
Groups and clusters form logical collections of Cisco Unified CallManager servers and their associated devices. Groups and clusters do not necessarily relate to the physical locations of any of their members. A group comprises a prioritized list of up to three Cisco Unified CallManager servers. You can associate each group with one or more device pools to provide call-processing redundancy. You use Cisco Unified CallManager Administration to define the groups, to specify which Cisco Unified CallManager servers belong to each group, and to assign a Cisco Unified CallManager group to each device pool. Each group must contain a primary Cisco Unified CallManager, and it may contain one or two backup Cisco Unified CallManager servers. The order in which you list the Cisco Unified CallManager servers in a group determines the priority order.
Cisco Unified CallManager groups provide both redundancy and recovery:
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Failover—Occurs when the primary Cisco Unified CallManager in a group fails, and the devices reregister with the backup Cisco Unified CallManager in that group.
•Fallback—Occurs when a failed primary Cisco Unified CallManager comes back into service, and the devices in that group reregister with the primary Cisco Unified CallManager.
Under normal operation, the primary Cisco Unified CallManager in a group controls call processing for all the registered devices (such as phones and gateways) that are associated with that group.
If the primary Cisco Unified CallManager fails for any reason, the first backup Cisco Unified CallManager in the group takes control of the devices that were registered with the primary Cisco Unified CallManager. If you specify a second backup Cisco Unified CallManager for the group, it takes control of the devices if both the primary and the first backup Cisco Unified CallManager servers fail.
When a failed primary Cisco Unified CallManager comes back into service, it takes control of the group again, and the devices in that group automatically reregister with the primary Cisco Unified CallManager.
Reference:
http://www.cisco.com/en/US/docs/voice_ip_comm/uc_system/UC5.0.1/ipt_system_arch/COL_FFR.html
Hope it helps.
Anand
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