Cisco Cisco IPCC Web Option Leaflet

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, if deploying Unified CVP and Unified CM 7.0 or 
later, MCS-7845 servers have a total server limit of 1,000 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.

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.

For additional information on general call processing topics such as secondary TFTP servers and 
gatekeeper considerations, refer to the Cisco Unified Communications Solution Reference Network 
Design (SRND) guide, available at

Agent PGs can be deployed in a Unified CM cluster in either of the following ways:

The first method is to deploy an Agent PG for each pair of Cisco Unified CM subscriber nodes. In 
this case, each Unified CM subscriber node runs the CTI Manager service, and each Agent PG 
connects to a CTI Manager running on its corresponding Unified CM subscriber pair. The following 
diagram shows an example where four primary Unified CM subscribers are required and four 
backup Unified CM subscribers are deployed to provide 1:1 redundancy.

Another possible method is to deploy a single Agent PG for the entire Cisco Unified CM cluster. 
This type of deployment requires a single pair of Cisco Unified CM subscriber nodes running CTI 
Manager. Agent phone registration should be spread among all the Cisco Unified CM subscriber 

271

173

Primary

PG A

Backup

PG B

Primary

PG A

Backup

PG B

Primary

PG A

Backup

PG B

Primary

PG A

Backup

PG B