Sunday, February 5, 2012

Quick Note on H323 and MGCP call preservation

A quick summary on H323 and MGCP gateway call preservation behavior when it works with UCM:

MGCP (primary UCM dies)
PSTN <-- MGCP GW --> UCM <--> Phone
- enabled by default
- UCM dies, call preserved
- Call continues over the gateway
- supplementary services will not work
- new calls to MGCP gateway will not work until MGCP gateway re-registers with the backup UCM

H323 (primary UCM dies)
PSTN <-- H323 GW --> UCM <--> Phone
- Default has no call preservation
- configure with "call preserve" command
- will immediately begin to use backup UCM for supplementary service, outbound and inbound call routing

Branch voice gateway (WAN link down)
- SRST kicks in
- If MGCP, then MGCP fallback - fallback to H323 / SIP processing
- MGCP - Call drop immediately at the branch router
- H323 is recommended for branch router - call preservation feature after IOS 12.4(9T)

Call preservation is configured in the gateway with the following command syntax:

Router(config)#voice service voip
Router(conf-serv-h323)#call preserve

MGCP fallback is configured with the following command syntax:

Router(config)#ccm-manager fallback-mgcp
Router(config-app-global)#service alternate Default


Boris said...

"Branch voice gateway (WAN link down)
- MGCP - Call drop immediately at the branch router"

True for ISDN services only.

But in the CCIE Voice lab scope, you are correct.

Demarco said...

Enabling "call preserve" under "voip service voip" activates call preservation for all IP calls. There's also the possibility to activate call preservation for specific dial peers, through the use of voice classes.