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.

Example 13-11. Preserving H.323 Calls on SRST Fallback

Miami(config)#voice service voip
Miami(conf-voi-serv)#h323
Miami(conf-serv-h323)#no h225 timeout keepalive
 

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.

Example 13-12. Preserving H.323 Calls on CallManager Failover

Miami(config)#voice service voip
Miami(conf-voi-serv)#h323
Miami(conf-serv-h323)#call preserve
 

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.

Secure SRST

Part I: Voice Gateways and Gatekeepers

Gateways and Gatekeepers

Part II: Gateways

Media Gateway Control Protocol

H.323

Session Initiation Protocol

Circuit Options

Connecting to the PSTN

Connecting to PBXs

Connecting to an IP WAN

Dial Plans

Digit Manipulation

Influencing Path Selection

Configuring Class of Restrictions

SRST and MGCP Gateway Fallback

DSP Resources

Using Tcl Scripts and VoiceXML

Part III: Gatekeepers

Deploying Gatekeepers

Gatekeeper Configuration

Part IV: IP-to-IP Gateways

Cisco Multiservice IP-to-IP Gateway

Appendix A. Answers to Chapter-Ending Review Questions

Index



Cisco Voice Gateways and Gatekeepers
Cisco Voice Gateways and Gatekeepers
ISBN: 158705258X
EAN: 2147483647
Year: 2004
Pages: 218

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