MGCP Fallback Usage
A public switched telephone network (PSTN) gateway can use MGCP gateway fallback configured as an individual feature if H.323 or SIP is configured as a backup service. SRST and MGCP fallback must be configured on the same gateway with Cisco IOS Software Release 12.2(11)T or later if this single gateway will provide SRST fallback service to phones and MGCP gateway fallback.
Although MGCP gateway fallback is most often used with SRST to provide gateway functions to IP Phones in SRST mode, it can also be used as a standalone feature. For example, when a call using a T-1 channel-associated signaling (CAS) interface controlled by MGCP exists during a CUCM failure, connectivity to the PSTN or another private branch exchange (PBX) can be preserved by MGCP gateway fallback. An MGCP fallback standalone configuration can also be used to allow analog interfaces that are controlled by SCCP to stay in service even when the WAN connection to the CUCM is down.
MGCP gateway fallback preserves active calls from remote-site IP Phones to the PSTN when analog or CAS protocols are used. For ISDN protocols, call preservation is impossible because Layer 3 of the ISDN stack is disconnected from the MGCP call agent and is restarted on the local Cisco IOS gateway. This means that for active ISDN calls, all call-state information is lost in cases of switchover to fallback operation.