During normal operation, Cisco IP phones exchange keepalive messages with their configured Cisco CallManagers. SRST is a licensed feature available on Cisco voice routers that allows Cisco IP phones to register with the gateway if these keepalive messages are lost. Cisco IP phones exchange the keepalive messages every 30 seconds by default. If three keepalive messages are missed, the IP phone initiates registration with the next device in its CallManager list. If the IP phone cannot reach a Cisco CallManager, it initiates registration with the gateway.
When the gateway receives a registration request from an IP phone, it acknowledges the request and enters SRST mode. The IP phone then sends its configuration settings to the SRST gateway. The gateway uses these settings to configure virtual dial peers on the SRST gateway for each IP phone directory number (DN), as shown in Figure 13-1.
Figure 13-1. SRST Registration Process
The time that it takes for an IP phone to initiate registration with the SRST gateway depends on what caused the failover. Three scenarios can lead to the phone registering with the SRST gateway:
Other factors that can affect the fallback time include the number of CallManagers configured, the phone types used, and the number of phones registering with the SRST router.
You can configure the keepalive timer in CallManager. This is a CallManager service parameter called StationKeepAliveInterval.
By default, the IP phone waits for 120 seconds after the CallManager connection is restored before unregistering with the SRST gateway. This prevents a flapping link or other intermittent problem from causing the IP phone to repeatedly unregister and reregister with SRST.
You can adjust the time that the IP phone waits to register to CallManager. You configure this time in CallManager by using the Device Pool>Connection Configuration Monitor menu option.
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