SRST Overview

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

Fallback Time

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:

• The CallManager process on the active CallManager is manually shut down This results in the TCP connection between the IP phone and the CallManager being closed. If no backup CallManager is available, the IP phone immediately attempts to register with the SRST gateway.
• IP connectivity between the IP phone and the CallManager is broken When the IP phone sends its first keepalive after the TCP connection is broken, it sends TCP retries for 20 to 25 seconds. Then it initiates registration with the SRST gateway.
• The CallManager process is locked In this case, the TCP connection is not closed. The IP phone waits for the keepalives to expire before initiating registration with the SRST gateway. This can take up to 90 seconds. This occurs when the server operating system (OS) is still functioning. If the entire server fails or is shut down, the TCP retries fail, as described in the previous bullet.

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.

Note

You can configure the keepalive timer in CallManager. This is a CallManager service parameter called StationKeepAliveInterval.

Restoral Time

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.