Cisco IP telephony systems are designed for high availability. In order to achieve this, the design must include redundancy for failover and rapid recovery. For recommendations and design assistance from Cisco Advanced Services, see Navigating the Road to Five Nines 
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The Unified Communications Manager plays the key role in maintaining call processing following a failure in an IP telephony environment. This topic describes the following high-availability features that are built into Unified Communications Manager:
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Unified Communications Manager Clusters
•Unified Communications Manager Redundancy Groups
Note The information in this topic applies to multiple Unified Communications Manager server clusters in a large enterprise environment and does not apply to Cisco Unified Communications Manager Business Edition. Cisco Unified Communications Manager Business Edition is a single-server solution designed for medium businesses up to a maximum of 500 employees and does not offer redundancy.
Unified Communications Manager Clusters
A cluster comprises a set of Unified Communications Manager servers (or nodes) that share the same database and resources. Unified Communications Manager servers can be configured to perform the following functions: database server, TFTP server, or application software server. You can dedicate a particular server to one function or combine several functions on one server, depending on the size of your network and the level of redundancy desired.
Each cluster can have only one database server (also called the first node) and usually one TFTP server (either separate or combined with another function). Cisco Systems recommends that large enterprise networks contain a dedicated Unified Communications Manager database server with other servers (called subsequent nodes) running the Unified Communications Manager application software. The Unified Communications Manager application software performs all call control, including signaling of endpoints, feature invocation, and calling restrictions. Large-scale networks typically use paired redundant application software servers, running in an active-active configuration, with endpoints evenly distributed across the two servers. The TFTP server provides configuration files for the endpoint devices and the associated firmware loads. Large enterprise networks typically use redundant TFTP servers.
Unified Communications Manager Redundancy Groups
A redundancy group comprises a prioritized list of up to three Unified Communications Manager servers. You can associate each group with one or more device pools to provide call processing redundancy. Each group must contain a primary Unified Communications Manager, and it may contain one or two backup Unified Communications Manager servers. If the primary Unified Communications Manager fails for any reason, the first backup Unified Communications Manager in the group takes control of the devices that were registered with the primary Unified Communications Manager. If you specify a second backup Unified Communications Manager for the group, it takes control of the devices if both the primary and the first backup Unified Communications Manager servers fail.
When a failed primary Unified Communications Manager comes back into service, it takes control of the group again, and the devices in that group automatically reregister with the primary Unified Communications Manager.
Keepalive Mechanism
A keepalive mechanism is an essential part of an IP telephony solution. Keepalives ensure that endpoints (typically phones and gateways) retain their communications path to a Unified Communications Manager server. Keepalives not only determine when the primary Unified Communications Manager server is no longer available, they also determine when the site has become completely isolated from a centralized call control system and must revert to some form of remote survivability capability such as Cisco Unified SRST. Keepalives avoid delays in establishing a call caused by searching for an available Unified Communications Manager server.