Call Preservation

One of the primary decision factors in selecting which gateway protocol to run is the preservation of calls during a failover situation. As discussed previously, MGCP gateways preserve active calls when an IP phone fails over from its primary CallManager to its secondary CallManager. Failover from CallManager to SRST is a little different.

For H.323 gateways, active calls are preserved until the H225 keepalive timer expires. You can disable the H225 keepalive timer so that active calls through an H.323 gateway are preserved indefinitely. Example 13-11 demonstrates how to disable the H225 timer.

For MGCP gateways, call preservation depends on the type of circuit. For analog or CAS circuits, calls are preserved on failover from CallManager to SRST. For ISDN circuits, active calls are dropped on failover. Technically, the call is dropped when you initiate MGCP Gateway Fallback. This is because the D channel of the ISDN circuit is backhauled to CallManager. When MGCP Gateway Fallback is initiated, the gateway tears down and reestablishes the D channel, resulting in all active calls being dropped. Calls are also dropped when communication is reestablished with CallManager.

Two of the primary reasons for selecting MGCP are call preservation and not having to build the dial plan in each gateway. Cisco IOS 12.4(4)XC introduced a new feature that allows calls through an H.323 gateway to be preserved when an IP phone fails over its primary CallManager to its secondary CallManager. This feature requires CallManager 4.1(3)SR1. Example 13-12 illustrates the configuration for preserving H.323 calls on a CallManager failover.

As discussed earlier, each gateway needs appropriate dial peers to function in SRST mode. Because H.323 call preservation is more robust for SRST, and the dial plan must be configured, most people select H.323 for their remote gateways.