Installing Backup CallManagers for Redundancy
To ensure redundancy, at least two CallManager servers must be installed.The
first one is the primary CallManager, which is used to make all changes to the
users and the VoIP system in general.The second CallManager is the one that
users will actually authenticate and have call control made through.The primary
call manager should not be used for call control services.The reason is that call
changes to the system are made on the primary server, which is reflected in the
MS SQL Server on CallManager.This SQL Server then propagates the database
changes pushed out to the secondary CallManagers on a regular basis. If users
were to authenticate against the primary CallManager while changes were taking
place, unpredictable results would occur.
Cisco claims the CallManager solution can support 2500 users per
CallManager server.The reality is that this number of users will choke most
infrastructures long before CallManager overloads itself, even with a primary and
secondary CallManager. Such massive utilization is where a distributed
CallManager solution comes into its best usage. However, it is possible a Gigabit
Ethernet backbone serving the CallManager solution as a whole can virtually
eliminate this bottleneck of Fast Ethernet. But if you upgrade the backbone,
don’t forget to upgrade the CallManager server to also support Gigabit Ethernet
lest you simply re-create the bottleneck.