MGCP fallback provides call control functionality using application alternate H.323 operation when a gateway is configured for MGCP in CUCM. An MGCP gateway registers with CUCM like SCCP/SIP phones. MGCP fallback enables a gateway to act as local call control when the CUCM server to which the remote site phones and gateway register is offline or WAN connectivity is lost (in which case, Cisco Unified SRST kicks in and offers call control functionality). This is specifically useful when a centralized call control using MGCP is desired for all voice gateways.
Figure 5-8 shows MGCP fallback (that is, MGCP application to default application H.323).
Figure 5-8 MGCP Fallback Operation
To configure MGCP fallback and enable default H.323 application-based SRST, perform the following steps:
- Step 1. Configure SRST reference in CUCM and assign it to the appropriate CUCM device pool (refer to the CUCM SRST Configuration section).
- Step 2. Configure the Call Forward Unregistered (CFUR) internal and external destinations on line of the remote site phones to an E.164 number (for example, a shared line on the main site or voicemail number). This is accomplished by going to Device > Phone, selecting a phone, selecting a line, and setting CFUR.
- Step 3. Configure the MGCP fallback and Cisco Unified SRST on the remote site gateways and implement the SRST dial plan on the remote site gateways. MGCP fallback configuration is shown in Example 5-6.
Example 5-6 MGCP Fallback Configuration
MGCP-Router(config)# ccm-manager fallback-mgcp MGCP-Router(config)# application MGCP-Router(config-app)# global MGCP-Router(config-app-global)# service alternate default
At this time, the IOS router is ready for fallback to H.323 application (defined as service alternate default in Example 5-6). Upon loss of connectivity (server/CallManager service is unavailable or WAN connection is down) to the CUCM server, the phones register with the gateway and continue to operate as normal (in SRST mode) until the link or CUCM service is restored.