Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.


Cluster sizing

I`m trying to locate the size per server plus the overall cluster size for a 7825 server. I can locate info. on 7835, 7845 servers but not for a 7825 for a CUCM 6.X - thanks

Hall of Fame Super Red

Re: Cluster sizing


Here is a clip from the 6.x SRND that discusses these questions;

With Cisco Unified CM 6.x, a cluster may contain as many as 20 servers, of which a maximum of eight may run the Cisco CallManager Service that provides call processing. The other servers may be configured as a dedicated database publisher, dedicated Trivial File Transfer Protocol (TFTP) server, or music on hold (MoH) servers. Media streaming applications (conference bridge or media termination point) may also be enabled on a separate server that registers with the cluster.

When deploying a cluster with Cisco MCS 7815 or equivalent servers, there is a maximum limit of two servers in a cluster: one as the publisher, TFTP server, and backup call processing server, and the other as the primary call processing server. A maximum of 500 phones is supported in this configuration with Cisco Unified CM 6.x on a Cisco MCS 7815. When deploying a two-server cluster with higher-capacity servers, Cisco recommends that you do not exceed 1250 users in the cluster. Above 1250 users, a dedicated publisher and separate servers for primary and secondary call processing services is recommended, thus increasing the number of servers in a cluster.

It is also possible to deploy a single-server cluster with an MCS 7825 or greater servers. With an ***MCS 7825 or equivalent server, the limit is 500 users; with a higher-availability server, the single-server cluster should not exceed 1000 users. In a single-server configuration, there is no redundancy unless Survivable Remote Site Telephony (SRST) is also deployed to provide service during periods when the Unified CM is not available. Cisco does not recommend a single-server deployment for production environments. The load balancing option is not available when the publisher is a backup call processing subscriber.

From the 6.x SRND;

Hope this helps!


Re: Cluster sizing


As far as my understanding goes, if I deploy 1:1 redundancy mode and have a dedicated Publisher not doing the call control function I can have up to 2000 IP Phones with 8 subscribers.

Rob can you please verify this.

-> Sushil


Re: Cluster sizing

Thanks for the info. I`m trying to put together a spread sheet on the size of the servers required along the lines

7825 up to 1250 users in a cluster

7835 above 2500 users in a cluster

7845 up to 5000 users in a cluster

Is this a good guide line?

At a min. there will always be two servers for redundancy regardlesss of the server spec.

Hall of Fame Super Red

Re: Cluster sizing

Hi Sushil/IPT,

These are excellent questions! Here are the base numbers from another section of the SRND;

Maximum Number of Devices per Server Platform

Cisco MCS 7845 (All supported models)


Cisco MCS 7835 (All supported models)


Cisco MCS 7825 (All supported models)


Cisco MCS 7815 or MCS 7816 (All supported models)


From this excellent section of the 6.x SRND;

Unified CM Platform Capacity Planning

This is a great tool, talk to your SE or Cisco Tac to gain access :)

The Cisco Unified Communications Manager Capacity Tool (Unified CMCT) enables you to calculate the capacity of the Unified CM system for various configurations.

The Cisco Unified CM Capacity Tool is available at

Hope this helps!


Re: Cluster sizing


Just an update -

MCS 7816 supports 500 users.

-> Sushil

CreatePlease login to create content