CUCM Call-Processing Redundancy Best CIsco CCSP Coaching in Gurgaon

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A CUCM cluster is a group of physical servers working as a single IP PBX system. With
CUCM 6.0, a cluster may contain up to 20 servers, of which a maximum of 8 servers may
run the Cisco CallManager service performing call processing in a cluster. Other servers
can be used as TFTP servers or provide media resources such as software conference
bridges or music on hold (MOH).
CUCM call-processing redundancy is implemented by grouping servers running the Cisco
CallManager service into CUCM groups. A CM group is a prioritized list of one or more
call-processing servers. Figure 2-5 shows this 1:1 redundancy design.
The following rules apply for the CM groups:
■ Multiple CM groups can exist in the same cluster.
■ Each call-processing server can be assigned to more than one CM group.
■ Each device has to have a CM group assigned that will determine the primary and
backup servers to which it can register.
46 Chapter 2: Deployment Models
Figure 2-5 1:1 Redundancy Design
Cisco IP Phones register with their primary server. When idle, the Cisco IP Phones and
CUCM exchange signaling application keepalives. In addition, Cisco IP Phones establish
a TCP session with their secondary server and exchange TCP keepalives. When the
connection to the primary server is lost (no keepalives received), the Cisco IP Phone
registers to the secondary server. The Cisco IP Phone will continuously try to reestablish
a connection with the primary server; if successful, the Cisco IP Phone will reregister with
the primary server.
A 1:1 CUCM call-processing redundancy deployment design guarantees that Cisco IP
Phone registrations will never overwhelm the backup servers. Multiple primary servers can
fail concurrently, and the cluster would still be fully operational. The 1:1 design has an
increased server count compared to other redundancy design models. This design is not the
most cost-effective, but it offers the highest level of fault tolerance.
Each cluster must also provide a TFTP service. The TFTP service is responsible for
delivering IP phone configuration and firmware files to telephones, along with streamed
media files, such as MOH and ring files; therefore, the server running the TFTP service can
Cisco MCS 7845
Publisher and
TFTP Server
Scenario Three:
30,000 IP Phones
1 to 7500
7501 to
15,000
15,001 to
22,500
22,501 to
30,000
Backups
Cisco MCS 7845
Publisher and
TFTP Server
Scenario Two:
15,000 IP Phones
1 to 7500
7501 to
15,000
Backups Backups
Cisco MCS 7845
Publisher and
TFTP Server
(Not Req. < 1000)
Scenario One:
7500 IP Phones
Primary
1 to 7500
Backup
Primary
Secondary/Backup
IP
CUCM Call-Processing Redundancy 47
experience a considerable network and processor load. Depending on the number of
devices that a server is supporting, you can run the TFTP service on a dedicated server,
on the database publisher server, or on any other server in the cluster.
In this example, a Cisco 7845 Media Convergence Server (MCS) is used as the dedicated
database publisher and TFTP server. In addition, there are two call-processing servers
supporting a maximum of 7500 Cisco IP Phones (on the Cisco 7845 MCS platform). One
of these two servers is the primary server, and the other is a dedicated backup server. The
function of the database publisher and the TFTP server can be provided by the primary or
secondary call-processing server in a smaller IP telephony deployment (fewer than 1250 IP
phones with the MCS 7845). In this case, only two servers are needed in total.
When you increase the number of IP phones, you must increase the number of CUCM
servers that are required to support the telephones. Some network engineers may consider
the 1:1 redundancy design excessive, because a well-designed network is unlikely to lose
more than one primary server at a time. With the low possibility of server loss and the
increased server cost, many network engineers choose to use a 2:1 redundancy design,
as shown in Figure 2-6.
Figure 2-6 2:1 Redundancy Design
Cisco MCS 7845
Publisher and
TFTP Server
Scenario Three:
30,000 IP Phones
1 to 7500
7501 to
15,000
15,001 to
22,500
22,501 to
30,000
Backup
Cisco MCS 7845
Publisher and
TFTP Server
Scenario Two:
15,000 IP Phones
1 to 7500
7501 to
15,000
Backup Backup
Cisco MCS 7845
Publisher and
TFTP Server
(Not Req. < 1000)
Scenario One:
7500 IP Phones
Primary
1 to 7500
Backup
Primary
Secondary/Backup
IP
48 Chapter 2: Deployment Models
Although the 2:1 redundancy design offers some redundancy, there is the risk of overwhelming
the backup server if multiple primary servers fail. In addition, upgrading the
CUCM servers can cause a temporary loss of service because rebooting the CUCM servers
is required after the upgrade is complete.
Network engineers use this 2:1 redundancy model in most IP telephony deployments
because of the reduced server costs. If a Cisco MCS 7845 is used (shown in the figure), that
server is equipped with redundant, hot-swappable power supplies and hard drives. It is
unlikely that multiple primary servers will fail at the same time, which makes the 2:1
redundancy model an attractive option for most businesses.
Cisco recommends dedicating a TFTP server in any cluster that exceeds 1250 phones. In a
large cluster, Cisco recommends dedicating two servers to TFTP functionality.
As shown in the first scenario, when using no more than 7500 IP phones, there is no saving
in the 2:1 redundancy design compared to the 1:1 redundancy design (simply because there
is only a single primary server).
In the second scenario of Figure 2-6 with up to 15,000 IP phones, there are 2 primary
servers (each serving 7500 IP phones) and 1 secondary server. As long as only one primary
server fails, the backup server can take over. If both primary servers fail, the backup server
could serve only half of the IP phones.
The third scenario in Figure 2-6 illustrates a deployment with 30,000 IP phones. Four
primary servers are required to facilitate this number of IP phones. For each pair of primary
servers, there is one backup server. As long as no more than two servers fail, the backup
servers can take over, and all IP phones will operate normally.