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Cisco IP Telephony Solution Reference Network Design
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Chapter 6 Call Processing
Gatekeeper Considerations
Load Balancing
An additional benefit of using the 1:1 redundancy scheme is that it enables you to balance the devices
over the primary and backup server pairs. Normally a backup server has no devices registered unless its
primary is unavailable.
With load balancing, you can move up to half of the device load from the primary to the secondary
subscriber by using the Cisco CallManager redundancy groups and device pool settings. In this way, you
can reduce by 50% the impact of any server becoming unavailable.
To plan for 50/50 load balancing, stay within the device weight limits for a single server to allow for the
failover scenario. For example, MCS 7835 servers have a total server limit of 5000 device weight units
and 2500 IP phones. In a 1:1 redundancy pair, you can split the load between the two subscribers,
configuring each subscriber with 2500 device weight units and 1250 IP phones. (See the configuration
for 2500 IP phones in
Figure 6-3
.)
To provide for failover conditions when only one server is active, make sure that total device weight
units, IP phones, CTI limits, and so on, for the redundant pair do not exceed the limits allowed for a
single server.
Secondary TFTP Server
For additional load balancing and redundancy, you can install a second Trivial File Transfer Protocol
(TFTP) server in the cluster. The TFTP server facilitates the downloading of configuration files, device
loads (operating code), and ring types. You should configure the secondary server for TFTP only, with
the Cisco CallManager service turned off so that no phones can register with that server. Once the
Cisco CallManager service is running on another server in that cluster, the intra-cluster communications
will populate the database, and all of the phone configurations will be created and placed into memory
on the TFTP servers.
Pay special attention to the secondary TFTP server if you have to change the phone load for a specific
type of phone in the cluster, but you are not upgrading the Cisco CallManager software at the same time.
In that case, be sure to copy the new phone load manually to each TFTP server.
With Cisco CallManager Release 3.3 and later, phone configuration files are not stored by default on the
hard drive of the TFTP server, as in earlier versions of Cisco CallManager. By default, all phone
configuration files are created and placed into RAM on the TFTP servers. You can change this default
setting to place the phone configuration files on the hard drive of the TFTP server, but doing so will
impact TFTP performance. Therefore, Cisco recommends that you do not change this default setting.
Gatekeeper Considerations
A Cisco IOS gatekeeper can provide call routing and call admission control for up to 100
Cisco CallManager clusters in a distributed call processing environment. You can also implement a
hybrid Cisco CallManager and toll bypass network by using Cisco IOS gatekeepers to provide
communication and call admission control between the H.323 gateways and Cisco CallManager.
Gatekeeper call admission control is a policy-based scheme requiring static configuration of available
resources. The gatekeeper is not aware of the network topology, so it is limited to hub-and-spoke
topologies.