Does the Customer Need Multiple Clusters?
You now need to decide how many clusters you need. A single cluster will support
a maximum of 10,000 IP phones, so your total of 2,800 falls well under this.
However, a cluster is only supported across fast LAN media and you have 20
remote sites supported across slow frame relay links.You have two options, the
first being to use multiple clusters, with one at the headquarters, and one at each
remote site.This could be seen as wasteful since each remote site would have at
least one CallManager server supporting only 40 clients. In addition, you would
lose a lot of CallManager functionality, as many CallManager features are not
supported between clusters.
A preferable option would be to use a single cluster located at the central site,
with each remote site defined as a location within CallManager.This would allow
CallManager to track and control voice traffic to remote sites without an expensive
and complex multicluster solution.
Cisco also offers a new IOS feature called Survivable Remote Site (SRS)
telephony.This feature allows a Cisco router at a remote site to automatically
detect a failure, and provide call processing functionality to IP phones for the
duration of the failure.When the link is restored, call processing automatically
switches back to Cisco CallManager.This ensures that local calls at small sites
without Cisco CallManager can still be processed in the event of a failure in the
link to the central site. Configuration is performed on the Cisco CallManager,
requiring little or no administration at the remote site. SRS is available on Cisco
2600 and 3600 routers, and Cisco Catalyst 4224 Access Gateway Switches.