benefit of using the 1:1 redundancy scheme is that it enables you to balance
the devices over the primary and backup subscriber pairs. Normally (as in the
2:1 redundancy scheme) a backup server has no devices registered unless its
primary is unavailable.
With load balancing,
you can move up to half of the device load from the primary to the secondary
subscriber by using the Unified CM redundancy groups and device pool settings.
In this way, you can reduce by 50% the impact of any Server Becoming
To plan for 50/50
load balancing, calculate the capacity of a cluster without load balancing and
then distribute the load across the primary and backup subscribers based on
devices and call volume. To allow for failure of the primary or the backup, the
total load on the primary and secondary subscribers must not exceed that of a
single subscriber. For example, if deploying Unified CVP and Unified CM 8.0 or
later releases, MCS-7845-H3 servers have a total server limit of 1000 Unified
CCE agents. In a 1:1 redundancy pair, you can split the load between the two
subscribers, configuring each subscriber with 500 agents. To provide for system
fault tolerance, make sure that all capacity limits are observed so that
Unified CCE agent phones, Unified IP phones, CTI limits, and so on, for the
subscriber pair do not exceed the limits allowed for a subscriber server.
devices and call volumes as equally as possible across all active subscribers.
For instance, distributing the Unified CCE agents, CTI ports, gateways, trunks,
voicemail ports, and other users and devices among all subscribers equally,
minimizes the impact of any outage.
information about general call processing topics such as secondary TFTP servers
and gatekeeper considerations, see the
Unified Communications System
Solution Reference Network Design (SRND) at