There Is Power in Numbers: Grouping Cisco CallManagers Together

When you create a cluster of CCM servers, part of the design process involves identifying the roles played by the various CCMs. For example, because the publisher plays such a vital role, you probably do not want IP phones registering with the publisher. Also, the IP phones' configuration files are stored on a Trivial File Transfer Protocol (TFTP) server, and you might wish to use the publisher for that function. Finally, you might wish to designate which CCMs are going to be backup CCMs for primary CCMs.

You also need to select your redundancy design approach, either 1:1 redundancy or 2:1 redundancy. With 1:1 redundancy, you designate a dedicated backup CCM server for each primary CCM server, to take over in the event of a primary CCM server failure. When you look at the bottom line, however, you might realize that a 1:1 redundancy approach can be cost prohibitive, due to the purchase of so many backup CCM servers.

To reduce costs, you might opt for a 2:1 redundancy model, where one CCM server acts as a backup for two primary CCM servers. Although a 2:1 redundancy model does offer cost savings, you sacrifice an extra layer of redundancy, as shown in Figure 4-3. Specifically, consider a scenario where CCM Server A serves as a backup to both CCM Server B and CCM Server C. Also assume that each CCM server supports 2500 IP phones. If CCM Server B failed, no big deal. The 2500 IP phones registered with CCM Server B simply reregister with CCM Server A (that is, the backup CCM server). The issue we might face, however, is in the unlikely event that both CCM Server B and C simultaneously fail. In that instance, a total of 5000 IP phones try to reregister with CCM Server A, which only supports 2500 IP phones. This potential oversubscription scenario reduces the overall fault tolerance of the 2:1 redundancy model.

Figure 4-3. Redundancy Design Options

Table 4-2 indicates the number of CCM servers Cisco recommends to support a specific number of IP phones (with an MCS platform supporting 2500 IP phones per server).

Table 4-2. Redundancy Models

IP Phones	1:1 Redundancy	2:1 Redundancy
Up to 2500	3 CCM Servers 1 Publisher and TFTP server (for 1o00 or more IP phones) 1 Primary CCM 1 Backup CCM	3 CCM Servers 1 Publisher and TFTP server (for 1000 or more IP phones) 1 Primary CCM 1 Backup CCM
Up to 5000	5 CCM Servers 1 Publisher and TFTP server 2 Primary CCMs 2 Backup CCMs	4 CCM Servers 1 Publisher and TFTP server 2 Primary CCMs 1 Backup CCM
Up to 10000	9 CCM Servers 1 Publisher and TFTP server 4 Primary CCMs 4 Backup CCMs	7 CCM Servers 1 Publisher and TFTP server 4 Primary CCMs 2 Backup CCMs