H.323 Call Preservation

     One of the big differences that used to exist between MGCP and H.323 was call preservation. However, H.323 has resolved that difference with "call preserve" (first integrated into IOS version 12.4(9)T) and it works quite well!

How to Setup H.323 Call Preservation

CUCM Configuration

1. In CUCM Administration, go to System > Service Parameters
2. From the Server drop down, select the Publisher server
3. From the Service drop down, Cisco CallManager
4. Click on the Advanced button on the top for additional parameters
5. Search for and set "Allow Peer to Preserve H.323 Calls" to True
6. Click Save

     This parameter determines whether CUCM allows the peer H.323 endpoint to try to preserve active H.323 calls by not sending a Release Complete message to the peer H.323 endpoint when the TCP connection is lost between CUCM and the far endpoint, such as a Skinny Client Control Protocol [SCCP] IP phone. When connectivity is lost, the H.225 and H.245 TCP connection is torn down but CUCM does not send the Release Complete message so that the peer H.323 endpoint can attempt to preserve the media path.

H.323 Gateway Configuration (Global)

• conf t
• voice service voip
• h323
• call preserve limit-media-detection

H.323 Gateway Configuration (Dial-Peer)

• conf t
• voice-class h323 10
• call preserve limit-media-detection
• dial-peer voice 1 voip
• voice-class h323 10

http://www.cisco.com/en/US/docs/ios/voice/command/reference/vr_c3.html#wp1181622

call preserve [limit-media-detection]

     The call preserve command activates H.323 VoIP call preservation for the following types of failures and connections:

Failure Types

• WAN failures that include WAN links flapping or degraded WAN links
• Cisco Unified CallManager software failure, such as when the ccm.exe service crashes on a Cisco Unified CallManager server.
• LAN connectivity failure, except when a failure occurs at the local branch

Connection Types

•  Calls between two CUCM controlled endpoints 
• During CUCM reloads 
• When a Transmission Control Protocol (TCP) connection between one or both endpoints and CUCM used for signaling H.225.0 or H.245 messages is lost or flapping  
• Between endpoints that are registered to different CUCM in a cluster and the TCP connection between the two CUCM is lost 
• Between IP phones and the PSTN at the same site
• Calls between Cisco IOS gateway and an endpoint controlled by a softswitch where the signaling (H.225.0, H.245 or both) flows between the gateway and the softswitch and media flows between the gateway and the endpoint. 
•  When the softswitch reloads. 
•  When the H.225.0 or H.245 TCP connection between the gateway and the softswitch is lost, and the softswitch does not clear the call on the endpoint 
•  When the H.225.0 or H.245 TCP connection between softswitch and the endpoint is lost, and the soft-switch does not clear the call on the gateway 
•  Call flows that involve a Cisco IP in IP (IPIP) gateway running in media flow-around mode that reload or lose connection with the rest of the network. 

     When bidirectional silence and RTP and RTCP inactivity detection are configured, they are enabled for all calls by default. To enable them for H.323 VoIP preserved calls only, you must use the call preserve command's limit-media-detection keyword. 

H.323 VoIP call preservation can be applied globally to all calls and to a dial peer. 

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