Clustering
Clustering will allow you to extend your support for IP devices from 2500 IP
telephones on an individual CallManager server, up to a potential 10,000 IP
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New World Technologies • Chapter 2 33
telephones within a single cluster. Clustering, as its name implies, is the process of
combining two or more CallManager servers into a logical unit known as a
group.A group consists of CallManager servers and their associated devices such as
IP telephones, gateways, and logical devices such as SoftPhones, a software-based
version of the IP telephone handset. (IP SoftPhones will be discussed further in
the IP telephony applications section to follow.) When the group concept is utilized,
all the CallManager servers share the same configuration database, so if one
CallManager server fails, the others already have the database, thus no manual
reconfiguration is required.The idea behind clustering has to do with providing
enough servers so that if one of them should fail, the other servers within the
cluster can take on the load of the failed server without compromising the level
of service to the end systems.
Cisco has outlined four primary roles a server can take on in the cluster:
Primary CallManager server
Backup CallManager server
Database publisher server
Trivial File Transfer Protocol (TFTP) server
The primary and backup CallManager servers are self-explanatory.The
database publisher server role is to maintain and distribute the master-configuration
database.A second but equally important task is the record warehousing of
call detail records (CDRs). A CDR is a record of the IP telephony call.This can
be used by other vendors’ software for traffic analysis and additional accounting
functions.The TFTP server role is used to provide the system image for devices
such as IP telephones and gateways.
How you structure your cluster is dependant on how many IP telephony
devices will be supported. Cisco has set the following design guidelines for
building your CallManager cluster. If you have fewer than 2500 IP telephones,
you will need two servers, one primary CallManager server, and one backup
CallManager/publisher/TFTP server. For 2500 IP phones, you will need three
servers, a primary CallManager server, a backup CallManager server, and a combined
Publisher/TFTP server. For 5000 IP phones, you will need four servers,
two primary CallManager servers, one backup CallManager server, and a combined
Publisher/TFTP server. For the maximum 10,000 IP telephones per
cluster, you will need four primary CallManager servers, two backup
CallManager servers, one database publisher server, and one TFTP server.
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34 Chapter 2 • New World Technologies
As we discussed in the introduction to this section, there are some limitations
you must take into consideration before implementing a cluster. An important
item to take into consideration is that a cluster cannot cross a WAN link. All
cluster servers must exist on the same LAN. Furthermore, the servers must be
interconnected at minimum by a 10 Mbps switched connection. Shared media is
not allowed in an AVVID cluster.This is to ensure the proper Quality of Service
(QoS) is maintained. Also, as stated earlier, a cluster is limited to 10,000 IP telephones.
A maximum of 100 clusters can be interconnected, allowing support for
up to 1,000,000 IP telephones within an organization. Figures 2.1 and 2.2
demonstrate clustering and failover protection.