Hi Bilal,
Yes, Callmanager uses clustering using 1 Publisher and Multiple Subscribers to accomplish full redundancy;
Here is some info about CCM Load Balancing;
Balanced Call Processing
After installing the Cisco Unified CallManagers that form a cluster, you should, as much as possible, evenly balance the call-processing load across the system by distributing the devices (such as phones, gateways, CTI route points, CTI ports, and route lists) among the various Cisco Unified CallManagers in the cluster. To distribute the devices, you configure Cisco Unified CallManager groups and device pools and then assign the devices to the device pools in a way that achieves the balance you want.
Cisco Unified CallManager groups and device pools represent logical groupings of devices that you can arrange in any way that you want. For ease of administration, make sure that all the devices in a group or pool share a common and easily identified characteristic, such as their physical location on the network.
You can also use Cisco Unified CallManager groups to establish redundancy (backup call processors) for the primary Cisco Unified CallManager in the group. A Cisco Unified CallManager group comprises an ordered list of up to three Cisco Unified CallManager servers. During normal operation, the first (primary) Cisco Unified CallManager in the group controls all device pools and devices that are assigned to that group. If the primary Cisco Unified CallManager in a group fails, control of the device pools and devices that are registered with the primary Cisco Unified CallManager transfers to the next Cisco Unified CallManager in the group list.
For example, assume a simplified system that comprises three Cisco Unified CallManagers in a cluster, with 300 existing Cisco Unified IP Phones and provisions to auto-register new phones as they are added later.
The configuration includes four Cisco Unified CallManager groups: group G1 assigned to device pool DP1, group G2 assigned to device pool DP2, group G3 assigned to device pool DP3, and group G4 assigned to device pool DP4. Group G4 serves as the default group for devices that auto-register.
Unified CM1 serves as the primary Cisco Unified CallManager for the devices in DP1 and DP2, first backup for DP3, and second backup for the devices in DP4.
Unified CM2 serves as the primary Cisco Unified CallManager for the devices in DP3 and DP4, first backup for DP1, and second backup for the devices in DP4.
Unified CM3 serves as the first backup Cisco Unified CallManager for the devices in DP2 and DP4 and second backup for the devices in DP1 and DP3.
From this good doc;
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00806b036a.html#wp1025126
Cisco Unified CallManager Groups
A Cisco Unified CallManager group comprises a prioritized list of up to three Cisco Unified CallManagers. The first Cisco Unified CallManager in the list serves as the primary Cisco Unified CallManager for that group, and the other members of the group serve as secondary (backup) Cisco Unified CallManagers.
Cisco Unified CallManager groups associate with devices through device pools. Each device belongs to a device pool, and each device pool specifies the Cisco Unified CallManager group for all of its devices.
Cisco Unified CallManager Groups:
Call processing load balancing You can configure device pools and Cisco Unified CallManager groups to distribute the control of devices across multiple Cisco Unified CallManagers.
Device Pools
Device pools provide a convenient way to define a set of common characteristics that can be assigned to devices.
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a008073ee5d.html#wp1059777
Hope this helps!
Rob
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