Gatekeepers can provide many of the same benefits in IP Telephony networks using CallManager. The gatekeeper can provide CAC and call routing between CallManager clusters, Cisco CallManager Express (CME), and H.323 gateways that are connected to legacy PBX systems.
As discussed, the gatekeeper is not aware of network topology, so it is limited to network hub-and-spoke topologies for CAC. When the gatekeeper is used for CAC with a CallManager-distributed call processing model, it is usually placed at the hub of the network.
When you are configuring the bandwidth available in the gatekeeper, be sure to consider the codec in use, as specified in the CallManager Region configuration. Include the bandwidth required for the voice codec in use and video bandwidth requirements, as shown in the Region configuration for CallManager 4.0 and higher systems.
In either a multicluster single campus or a distributed call processing model, you must set up connections between CallManager clusters. One type of connection is known as an intercluster trunk and is the simplest type of trunk that CallManager supports.
You must define intercluster trunks each way between clusters. You can also set them up among a maximum of three separate servers in the destination cluster for redundancy and load balancing. This means that as many as six trunks might need to be configured between two clusters.
This works for smaller deployments, but it becomes administratively difficult to manage the full mesh configuration that is required for a larger number of clusters.
It is recommended that gatekeeper-controlled intercluster trunks (or H.225 gatekeeper-controlled trunks on CallManager 3.2 and higher systems) be used to simplify the administration. Gatekeeper-controlled trunks also provide faster failover if a subscriber server in a cluster becomes unreachable. It is only necessary to configure a single gatekeeper-controlled trunk from each cluster to the gatekeeper. You can assign up to three subscriber servers to the single trunk for load balancing and redundancy. Even though you configure only a single trunk, the gatekeeper registers multiple trunks, one from each assigned subscriber server.
After you configure the trunk, you can direct route patterns to use it. The gatekeeper provides E.164 number resolution for the CallManager cluster. You can use this same mechanism to integrate CallManager clusters into a hybrid H.323 network environment containing CME or H.323 gateways that are connected to PBX systems.
It is recommended that you register each CallManager cluster to a separate zone on the gatekeeper.
For more information on using gatekeepers with CallManager, see the Solution Reference Network Design (SRND) guide available at http://www.cisco.com/go/srnd.
Part I: Voice Gateways and Gatekeepers
Gateways and Gatekeepers
Part II: Gateways
Media Gateway Control Protocol
Session Initiation Protocol
Connecting to the PSTN
Connecting to PBXs
Connecting to an IP WAN
Influencing Path Selection
Configuring Class of Restrictions
SRST and MGCP Gateway Fallback
Using Tcl Scripts and VoiceXML
Part III: Gatekeepers
Part IV: IP-to-IP Gateways
Cisco Multiservice IP-to-IP Gateway
Appendix A. Answers to Chapter-Ending Review Questions