5.3 MGCP Fallback Operation

his topic describes how MGCP fallback works.

MGCP Gateway Fallback: Normal Situation

  • MGCP gateways register with Cisco Unified Communications Manager.
  • MGCP gateways exchange keepalive messages with the central Cisco Unified Communications Manager across the WAN.
  • Cisco Unified Communications Manager is the MGCP call agent.

The figure illustrates the normal operation of an MGCP gateway.

MGCP gateway fallback is a feature that improves the reliability of MGCP branch networks. A WAN link connects the MGCP gateway at a remote site to the Cisco Unified Communications Manager at a central site, which is the MGCP call agent. If the WAN link fails, the fallback feature keeps the gateway working as an H.323 or SIPgateway and rehomes back to the MGCP call agent when the WAN link is active again. MGCP gateway fallback works along with the Cisco Unified SRST feature.

Cisco IOS gateways can maintain links to up to two backup Cisco Unified Communications Manager servers in addition to a primary Cisco Unified Communications Manager. This redundancy enables a voice gateway to switch over to a backup server if the gateway loses communication with the primary server. The secondary backup server takes control of the devices that are registered with the primary Cisco Unified Communications Manager.

The tertiary backup takes control of the registered devices if both the primary and secondary backup Cisco Unified Communications Manager systems fail. The gateway preserves existing connections during a switchover to a backup Cisco Unified Communications Manager.

When the primary Cisco Unified Communications Manager server becomes available again, control reverts to that server. Reverting to the primary server can occur in several ways: immediately, after a configurable amount of time, or only when all connected sessions are released.

MGCP Gateway Fallback: Switchover

This section describes what happens when the MGCP gateway loses connectivity with all its call control agents (primary and backup Cisco Unified Communications Manager servers).

  • If the WAN link fails, MGCP gateways lose contact with Cisco Unified Communications Manager.
  • MGCP gateway tries to connect to the fallback Cisco Unified Communications Manager.
  • MGCP gateway falls back to its default application (H.323 or SIP).

The MGCP gateway performs a switchover to its default technology when the keepalives between Cisco Unified Communications Manager and the Cisco MGCP gateway are missing.

The MGCP gateway fallback feature provides the following functionality:

  • MGCP gateway fallback support: All active MGCP analog, E1 CAS, and T1 CAS calls are maintained during the fallback transition. Callers are unaware of the fallback transition, and the active MGCP calls are cleared only when the callers complete their calls (hang up). Active MGCP PRI backhaul calls are released during fallback. Any transient MGCP calls (that is, calls that are not in the connected state) are cleared at the onset of the fallback transition and must be attempted again later.
  • Basic connection services in fallback mode: Basic connection services for IP telephony traffic that passes through the gateway are provided. When the local MGCP gateway transitions into fallback mode, the default H.323 session application assumes responsibility for managing new calls. Only basic two-party voice calls are supported during the fallback period. When a user completes an active MGCP call, the MGCP application processes the on-hook event and clears all call resources.

MGCP Gateway Fallback: Switchback

This section describes what happens when the connectivity with Cisco Unified Communications Manager is restored.

  • When the WAN link is restored, MGCP gateway starts the rehome function.
  • MGCP gateway reregisters with Cisco Unified Communications Manager.
  • Gateway switches back to normal MGCP application mode.

The switchback or rehome mechanism is triggered by the re-establishment of the TCP connection between Cisco Unified Communications Manager and the Cisco MGCP gateway.

The MGCP gateway fallback feature provides the rehome functionality to switch back to MGCP mode.

The rehome function in gateway fallback mode detects the restoration of a WAN TCP connection to the primary Cisco Unified Communications Manager server. When the fallback mode is in effect, the affected MGCP gateway repeatedly tries to open a TCP connection to a Cisco Unified Communications Manager server that is included in the prioritized list of call agents. This process continues until a Cisco Unified Communications Manager server in the prioritized list responds. The TCP open request from the MGCP gateway is recognized, and the gateway reverts to MGCP mode. The gateway sends an RSIP message to begin registration with the responding Cisco Unified Communications Manager.

All currently active calls that are initiated and set up during the fallback period are maintained by the default H.323 session application, except ISDN T1 and E1 PRI calls. Transient calls are released. After rehoming occurs, the new Cisco Unified Communications Manager assumes responsibility for controlling new IP telephony activity.

MGCP Gateway Fallback Process

This section shows all the steps of the MGCP fallback process before, during, and after a WAN connectivity issue.

The MGCP gateway maintains a remote connection to a centralized Cisco Unified Communications Manager cluster by sending MGCP keepalive messages to the Cisco Unified Communications Manager server at 15-second intervals.

If the active Cisco Unified Communications Manager server fails to acknowledge receipt of the keepalive message within 30 seconds, the gateway attempts to switch over to the next available Cisco Unified Communications Manager server.

If none of the Cisco Unified Communications Manager servers respond, the gateway switches into fallback mode and reverts to the default H.323 session application for basic call control. H.323 is a standardized communication protocol that enables dissimilar devices to communicate with each other by using a common set of codecs, call setup and negotiating procedures, and basic data-transport methods. The gateway processes calls on its own by using H.323 until one of the Cisco Unified Communications Manager connections is restored.

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