MGCP Gateway Fallback

In an H.323 or SIP gateway, the router controls the voice ports locally, and a CallManager failure is automatically handled by rerouting calls to lower preference dial peers. In an MGCP gateway, CallManager controls the voice ports. If the IP phones cannot communicate with their configured CallManagers, it is likely that the MGCP gateway has also lost communication with the cluster. For IP phones to be able to make calls using the voice ports, MGCP must relinquish control. To accommodate this, MGCP gateways also send keepalive messages to their configured CallManagers. You can configure an MGCP gateway to "fall back" from MGCP mode to another call control application when these keepalive messages are lost. This is often referred to as falling back to H.323 mode, but this is not technically correct. SRST is typically invoked when the IP WAN fails, so any H.323 or SIP dial peers that are configured to route calls to CCM will most likely also be unavailable. Secondary, or lower preference, dial peers that point to other destinations, however, can route calls successfully. The gateway is actually falling back to an application, or Toolkit Command Language (Tcl) script, loaded in the gateway that determines the way calls are processed. All Cisco voice gateways have an application named default that allows the gateway to process calls using locally defined POTS dial peers. You can load other applications to provide this capability, but that topic is beyond the scope of this book.