Configuring Intercluster Trunks

Although the SQL 2000 database shared between CallManagers in your cluster provides a listing of all extensions that a user can dial within the cluster, you must manually add connections and route patterns to remote clusters before they can be reached. The connection type that acts as the road between CallManager clusters is the intercluster trunk. Depending on the size of your IP telephony network, you can create trunks directly between CallManager clusters or you can create trunks to connect to an H.323 gatekeeper.

Note

You can create a network of enormous size using only intercluster trunks; however, because the trunks are a full-mesh configuration, it can become increasingly difficult to manage the larger your network gets. Intercluster trunks also lack the ability to perform call admission control, which could result in the WAN becoming swamped with VoIP traffic.

A gatekeeper provides call admission control by using the H.225 Registration, Admission, and Status (RAS) protocol message set that is used as a central point for call admission control, bandwidth allocation, and dial pattern resolution (call routing). The gatekeeper provides these services for communications between Cisco CallManager clusters and H.323 networks. Note that the voice path is between the endpoints; no RTP audio travels through the gatekeeper.

You can configure gatekeepers and trunks in Cisco CallManager Administration to function in either of the following ways:

• Nongatekeeper-controlled trunks In this case, you explicitly configure a separate Intercluster Trunk for each remote device cluster that the local Cisco CallManager can call over the IP WAN, as shown in Figure 9-4. You also configure the necessary dial plan details to route calls to and from the various Intercluster Trunks. The Intercluster Trunks statically specify the IP addresses of the remote devices. To use this method, choose Device > Trunk and then choose Inter-Cluster Trunk (Non-Gatekeeper Controlled) in Cisco CallManager Administration.

  Figure 9-4. Nongatekeeper-Controlled Trunk Design

• Gatekeeper-controlled trunks In this case, a single intercluster trunk from each CallManager cluster suffices for communicating with all remote clusters, as shown in Figure 9-5. Similarly, you need only a single H.225 trunk to communicate with multiple H.323 gatekeeper-controlled endpoints. In this configuration, the gatekeeper can dynamically determine the appropriate IP address for the destination of each call to a remote device, and the local Cisco CallManager uses that IP address to complete the call.

  Figure 9-5. Gatekeeper-Controlled Trunk Design

  This configuration works well in large and smaller systems. For large systems in which many clusters exist, this configuration helps to avoid the configuration of individual Intercluster Trunks between each cluster. To use this method, choose Device > Trunk and choose Inter-Cluster Trunk (Gatekeeper Controlled) in Cisco CallManager Administration.

  If you configure gatekeeper-controlled trunks, Cisco CallManager automatically creates a virtual trunk device. The IP address of this device changes dynamically to reflect the IP address of the remote device as determined by the gatekeeper.

Complete these steps to configure a nongatekeeper-controlled intercluster trunk:

Note

Configuring gatekeeper-controlled intercluster trunks is covered in Chapter 14, "Implementing Multiple-Site Deployments."