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Cisco IP Telephony Solution Reference Network Design
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Chapter 6 Call Processing
Call Processing Redundancy
Note
Dial plan weight calculations are separate from device weight calculations. The subscriber servers and
the cluster must comply with both the device weight limit and the dial plan weight limit. If either limit
is exceeded, reduce the number of devices, line appearances, and so on, to achieve compliance.
A Cisco CallManager cluster with a very large dial plan containing many gateways, route patterns,
translation patterns, and partitions can take an extended amount of time to initialize when the
Cisco CallManager Service is first started. If the system does not initialize within the default time, there
are service parameters that you can increased to allow additional time for the configuration to initialize.
For details on the service parameters, refer to the Cisco CallManager Administration Guide and System
Guide, available on Cisco.com.
Call Processing Redundancy
With all versions of Cisco CallManager, you can choose from the following redundancy configurations:
·
Two to one (2:1) -- For every two primary subscribers, there is one shared backup subscriber.
·
One to one (1:1) -- For every primary subscriber, there is a backup subscriber.
The 1:1 redundancy scheme allows upgrades with only the failover periods impacting the cluster. The
failover mechanism has been enhanced so that you can achieve failover rates for the IP phones of
approximately 100 registrations per second.
Cisco CallManager Release 3.3 supports up to eight subscribers (servers with the Cisco CallManager
service enabled), so you may have as many as four primary and four backup subscribers in a cluster.
The 1:1 redundancy scheme enables you to upgrade the cluster using the following method.
Step 1
Upgrade the publisher server.
Step 2
Upgrade dedicated TFTP and music on hold (MoH) servers.
Step 3
Upgrade all backup subscribers. This step will impact some users if 50/50 load balancing is
implemented.
Step 4
Fail-over the primary subscribers to their backups, and stop the Cisco CallManager service on the
primaries.
Step 5
Upgrade the primaries, then re-enable the Cisco CallManager service.
With this upgrade method, there is no period (except for the failover period) when devices are registered
to subscriber servers that are running different versions of the Cisco CallManager software. This factor
can be important because the Intra-Cluster Communication Signaling (ICCS) protocol that
communicates between subscribers can detect a different software version and shut down
communications to that subscriber. This action could potentially partition a cluster for call processing,
but SQL and LDAP replication would not be affected.
The 2:1 redundancy scheme allows for fewer servers in a cluster, but it can potentially result in an outage
during upgrades.