H.323 VoIP call preservation enhancements for WAN link failures sustains connectivity for H.323 topologies where signaling is handled by an entity that is different from the other endpoint, such as a gatekeeper that provides routed signaling or a call agent, such as the Cisco BTS 10200 Softswitch, Cisco PGW 2200, or Cisco CallManager, that brokers signaling between the two connected parties.
Call preservation is useful when a gateway and the other endpoint (typically an Cisco Unified IP phone) are collocated at the same site and the call agent is remote and therefore more likely to experience connectivity failures.
Calls between two Cisco Unified CallManager controlled endpoints
During Cisco Unified CallManager reloads
When a Transmission Control Protocol (TCP) connection between one or both endpoints and Cisco Unified CallManager used for signaling H.225.0 or H.245 messages is lost or flapping
Between endpoints that are registered to different Cisco Unified CallManagers in a cluster and the TCP connection between the two Cisco Unified CallManagers 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
Note that after the media is preserved, the call is torn down later when either one of the parties hangs up or media inactivity is detected. In cases where there is a machine-generated media stream, such as music streaming from a media server, the media inactivity detection will not work and the call may hang. Cisco Unified CallManager addresses such conditions by indicating to the gateway that such calls should not be preserved, but third-party devices or IPIP gateways would not do this.
Flapping is defined for this feature as the repeated and temporary loss of IP connectivity that can be caused by WAN or LAN failures. H.323 VoIP calls between a Cisco IOS gateway and Cisco Unified CallManager may be torn down when flapping occurs. When Cisco Unified CallManager detects that the TCP connection is lost, it clears the call and closes the TCP sockets used for the call by sending a TCP FIN, without sending an "H.225.0 Release Complete" or "H.245 End Session" message. This is called quiet clearing. The TCP FIN sent from the Cisco Unified CallManager could reach the gateway if the network comes up for a short duration, and the gateway will tear the call down. Even if the TCP FIN does not reach the gateway, the TCP keepalives sent from the gateway could reach Cisco Unified CallManager when the network comes up. Cisco Unified CallManager will send TCP RST messages in response to the keepalives as it has already closed the TCP connection. The gateway will tear down H.323 calls if it receives the RST message.
Configuration of H.323 VoIP call preservation enhancements for WAN link failures involves configuring the call preserve command. If you are using Cisco Unified CallManager you must enable the "Allow Peer to Preserve H.323 Calls" parameter from Cisco Unified CallManager’s Service Parameters window.
The call preserve command causes the gateway to ignore socket closure or socket errors on H.225.0 or H.245 connections for active calls, allowing the socket to be closed without tearing down calls using those connections.
Call preservation may be reported through Syslog, which optionally can be obtained through a simple network management protocol (SNMP) trap. New syslog messages are printed when call preservation is applied. An SNMP trap can be configured on this syslog message, so you can be notified when call preservation occurs on a gateway.
Preservation information is displayed through the show h323 calls preserved command. The following is an example of the command’s output:
CallID = 11EC , Calling Number = , Called Number = 3210000 , RemoteSignallingIPAddress=220.127.116.11 , RemoteSignallingPort=49760 , RemoteMediaIPAddress=18.104.22.168 , RemoteMediaPort=17910 , Preserved Duration = 262 , Total Duration = 562 , H225 FD = -1 , H245 FD = -1
The previous example represents one preserved call. One such display is provided per preserved call. The show h323 calls preserved displays active calls only. No history is output.
To obtain additional information about a call, you can also use the show call active voice command. Calls can be cleared with the clear call voice causecode command.