5.4 Cisco Unified SRST Operation

This topic describes how Cisco Unified SRST works.

Cisco Unified SRST Function: Normal Situation

  • Branch IP phones register with Cisco Unified Communications Manager.
  • IP phones exchange keepalive messages with the central Cisco Unified Communications Manager across the WAN.
  • Cisco Unified Communications Manager manages all call processing.

The figure illustrates the normal operation of remote phones.

Cisco Unified Communications Manager supports Cisco Unified IP phones at remote sites that are attached to Cisco multiservice routers across the WAN. The remote site IP phones register with Cisco Unified Communications Manager. Keepalive messages are exchanged between IP phones and the central Cisco Unified Communications Manager across the WAN. Cisco Unified Communications Manager at the main site manages the call processing for the branch IP phones.

Cisco Unified SRST Function: Switchover Signaling

This section describes what happens when remote IP phones lose connectivity to Cisco Unified Communications Manager.

  • If the WAN link fails, IP phones lose contact with Cisco Unified Communications Manager.
  • IP phones register with local gateway.
  • Gateway queries IP phones for configuration and it autoconfigures itself.

When Cisco Unified IP phones lose contact with Cisco Unified Communications Manager, they register with the local Cisco Unified SRST router to sustain the call-processing capability that is necessary to place and receive calls.

The Cisco Unified SRST configuration provides the Cisco Unified IP phones with the alternative call control destination of the Cisco Unified SRST gateway.

When the WAN link fails, the Cisco Unified IP phones lose contact with the central Cisco Unified Communications Manager but then register with the local Cisco Unified SRST gateway.

The Cisco Unified SRST gateway detects newly registered IP phones, queries these IP phones for their configuration, and then autoconfigures itself. The Cisco Unified SRST gateway uses SNAP technology to autoconfigure the branch office router to provide call processing for Cisco Unified IP phones that are registered with the router.

Cisco Unified SRST Function: Call Flow After Switchover

The figure illustrates what happens when the IP phones register with the Cisco Unified SRST router.

  • Calls between remote site phones survive switchover.
  • Remote site calls that use the local non-MGCP gateway survive switchover.
  • Gateway processes calls during a WAN failure.
  • Cisco Unified SRST features such as autoprovisioning and failover are supported.

Cisco Unified SRST ensures that Cisco Unified IP phones offer continuous service by providing call-processing support directly from the Cisco Unified SRST router, using a basic set of call-processing features.

The Cisco Unified SRST gateway uses the local PSTN breakout. Cisco Unified SRST features, such as call preservation, autoprovisioning, and failover, are supported.

During a WAN connection failure, when Cisco Unified SRST is enabled, a Cisco Unified IP phone displays a message informing the user that the phone is operating in Cisco Unified Communications Manager fallback mode. This message can be adjusted.

While in Cisco Unified Communications Manager fallback mode, Cisco Unified IP phones continue to send out keepalive messages to attempt to re-establish a connection with Cisco Unified Communications Manager at the main site.

Cisco Unified SRST Function: Switchback

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

  • WAN link is restored, and IP phones are able to re-establish contact with Cisco Unified Communications Manager.
  • Cisco Unified IP phones reregister with their primary Cisco Unified Communications Manager.
  • Call processing reverts to the primary Cisco Unified Communications Manager.

Cisco Unified IP phones attempt to re-establish a connection with Cisco Unified Communications Manager at the main site periodically when they are registered with a Cisco Unified SRST gateway.

The default time that Cisco Unified IP phones wait before attempting to re-establish a connection to a remote Cisco Unified Communications Manager is generally up to 120 seconds.

When the WAN link or connection to the primary Cisco Unified Communications Manager is restored, after a configured waiting behavior, the Cisco Unified IP phones reregister with the primary Cisco Unified Communications Manager. Three switchback methods are available on the Cisco IOS router: immediate switchback, graceful switchback (after all outgoing calls on the gateway are completed), or switchback after a configured delay. Once switchback is completed, call processing reverts to the primary Cisco Unified Communications Manager, and SRST returns to standby mode.

Cisco Unified SRST Process

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

It takes three times the keepalive period for a phone to discover that its connection to Cisco Unified Communications Manager has failed. The default keepalive period is 30 seconds.

If the IP phone has an active standby connection that is established with a Cisco Unified SRST router, the fallback process takes 10 to 20 seconds after the connection with Cisco Unified Communications Manager is lost. An active standby connection to a Cisco Unified SRST router exists only if the phone has a single Cisco Unified Communications Manager in its Cisco Unified Communications Manager group. Otherwise, the phone activates a standby connection to its secondary Cisco Unified Communications Manager.

Note

The time that it takes for an IP phone to fall back to the Cisco Unified SRST router can vary, depending on the phone type.

If a Cisco Unified IP phone has multiple Cisco Unified Communications Manager systems in its Cisco Unified Communications Manager group, the phone progresses through its list before attempting to connect with its local Cisco Unified SRST router. Therefore, the time that passes before the Cisco Unified IP phone eventually establishes a connection with the Cisco Unified SRST router increases with each attempt to contact a Cisco Unified Communications Manager. Assuming that each attempt to connect to a Cisco Unified Communications Manager takes about 1 minute, the Cisco Unified IP phone in question could remain offline for 3 minutes or more following a WAN link failure. You can reduce this time by setting the keepalive timer to a smaller value. You can configure the keepalive timer by using the Cisco CallManager service parameter that is named Station Keepalive Interval.

While in SRST mode, Cisco Unified IP phones periodically attempt to re-establish a connection with Cisco Unified Communications Manager at the main site. The default time that Cisco Unified IP phones wait before attempting to re-establish a connection to Cisco Unified Communications Manager is generally 120 seconds.