Gatekeepers help VoIP networks scale to large sizes. Companies that have geographically dispersed voice networks, or networks that have become so large that they are unwieldy, might opt to segment their network. In a CallManager network, you can create multiple clusters. In that case, you would need to configure a full mesh of connections over the IP WAN to link all the segments or clusters. You would need to configure dial information for every remote location on every gateway and CallManager cluster. A better alternative is to use gatekeepers. In a network that has gatekeepers, trunks are needed only to the gatekeeper, and the gatekeeper maintains remote endpoint information.
When you use gatekeepers, gateways and CallManagers register with their gatekeeper. Gatekeepers divide the network into "zones," or groups of devices that register with a particular gatekeeper. When an H.323 gateway receives a call that is destined to a remote phone, it queries the gatekeeper for the location of the endpoint. If the call is destined for a different zone, you can configure the gatekeeper to allow it only if sufficient bandwidth is available. In more complex networks, you can use a Directory gatekeeper to maintain information about all the zones. You can configure Cisco routers with the appropriate Cisco IOS as H.323 gatekeepers.
Figure 1-2 shows an example of a company that has three CallManager clusters and a gatekeeper. Each cluster has an intercluster trunk over the IP WAN to the gatekeeper. Each cluster is its own zone.
Figure 1-2. H.323 Gatekeeper Example
Gatekeeper functionality is part of the H.323 standard. A voice gatekeeper provides the following services:
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