SRST
MGCP gateways that lose connectivity to their primary Cisco Unified Communications Manager (CUCM) support call survivability by default, but the MGCP gateway cannot be used for any new inbound or outbound calls during a CUCM outage. During the CUCM outage, phones with existing calls will see an LCD message indicating CUCM is down and features are disabled (supplementary services). The MGCP gateway will re-register to a backup CUCM server when all calls have gone through normal call clearing at the end of a phone call. Let’s investigate an MGCP controlled branch router operation during a WAN link failure. SRST will allow the branch phones to re-register to the local SRST branch router, but the MGCP gateway functionality required to control the TDM interfaces on the router require an MGCP call agent (CUCM). MGCP fallback commands can be provisioned to allow the MGCP gateway to fallback to H.323/SIP call processing. MGCP fallback will allow the router to immediately revert to local call processing when there is a WAN outage, but all active PSTN calls will be dropped immediately at the branch router. H.323 gateways using the call preservation feature of IOS 12.4(9T) do not have this limitation. I recommend H.323 as a gateway protocol for remote branch offices due to this MGCP limitation. Call preservation is configured in the gateway with the following command syntax: Router(config)#voice service voip Router(conf-voi-serv)#h323 Router(conf-serv-h323)#call preserve MGCP fallback is configured with the following command syntax: Router(config)#ccm-manager fallback-mgcp Router(config)#application Router(config-app)#global Router(config-app-global)#service alternate Default In the next blog, we’ll continue covering SRST related technology.
