Cisco Cisco Unified Contact Center Enterprise 9.0(2) Design Guide

An additional 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 unavailable.

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 should 
not exceed that of a single subscriber. For example, MCS-7845 servers have a total server limit of 500 
Unified CCE agents. In a 1:1 redundancy pair, you can split the load between the two subscribers, 
configuring each subscriber with 250 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.

Cisco recommends the equal distribution of all devices and call volumes as much 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.

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