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CUCME in SRST mode enables routers to provide basic call-handling support for Cisco
Unified IP Phones if they lose connection to remote primary, secondary, and tertiary CUCM
installations or if the WAN connection is down.
When Cisco Unified SRST functionality is provided by CUCME, provisioning of phones
is automatic. Most CUCME features are available to the phones during periods of fallback,
including hunt groups, Call Park, and access to Cisco Unity voice messaging services using
SCCP. The benefit is that CUCM users gain access to more features during fallback without
any additional licensing costs.
CUCME in SRST mode offers a limited telephony feature set during fallback mode. If the
following features are required, basic SRST should be used, because these features are not
supported with CUCM in SRST mode:
• More than 240 phones exist during fallback service.
• Cisco VG248 Analog Phone Gateway support is required.
• Secure voice fallback during SRST fallback service.
• Simple, one-time configuration for SRST fallback service.
Cisco Unified SRST Operation
Figure 5-2 illustrates the function of Cisco Unified SRST.
Basic Cisco Unified SRST Usage 103
Figure 5-2 SRST Function in a Normal Situation
Main Site Remote Site
CUCM supports Cisco Unified IP Phones at remote sites attached to Cisco multiservice
routers across the WAN. The remote-site IP Phones register with CUCM. Keepalive
messages are exchanged between IP Phones and the central CUCM server across the WAN.
CUCM at the main site handles the call processing for the branch IP Phones. Note that
Cisco IP Phones cannot register SCCP or SIP through the PSTN to CUCM even if the
PSTN is functional, because SCCP and SIP must run over an IP network.
SRST Function of Switchover Signaling
When Cisco Unified IP Phones lose contact with CUCM (as shown in Figure 5-3) because
of any kind of IP WAN failure, they register with the local Cisco Unified SRST router
to sustain the call-processing capability that is necessary to place and receive calls.
Figure 5-3 SRST Function of Switchover Signaling
Main Site Remote Site
104 Chapter 5: Examining Remote-Site Redundancy Options
Cisco Unified SRST configuration provides the IP Phones with the alternative call control
destination of the Cisco Unified SRST gateway.
When the WAN link fails, the IP Phones lose contact with the central CUCM 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.
CAUTION In the event of a WAN failure, when the branch IP Phones register to the
SRST gateway, do not erase the settings on the phone. The phone settings, such as phone
numbers, were added to the phone from CUCM when the WAN was functional, and the
phone then takes its settings and uses them to register to the SRST router. If the phone
has no settings, it cannot register to SRST, and the phone will not be functional.
SRST Function of the Call Flow After Switchover
Cisco Unified SRST ensures that Cisco Unified IP Phones offer continuous service by
providing call-handling support directly from the Cisco Unified SRST router using a basic
set of call-handling features, as shown in Figure 5-4.
Figure 5-4 SRST Function of the Call Flow After Switchover
Main Site Remote Site
The Cisco Unified SRST gateway uses the local PSTN breakout with configured dial peers.
Cisco Unified SRST features such as call preservation, autoprovisioning, and failover are
supported.
Basic Cisco Unified SRST Usage 105
During a WAN connection failure, when Cisco Unified SRST is enabled, Cisco Unified IP
Phones display a message informing users that the phone is operating in CUCM fallback
mode. This message can be adjusted.
While in CUCM fallback mode, Cisco Unified IP Phones continue sending keepalive
messages in an attempt to reestablish a connection with the CUCM server at the main site.
SRST Function of Switchback
Figure 5-5 shows Cisco Unified IP Phones attempting to reestablish a connection over the
WAN link with CUCM at the main site periodically when they are registered with a Cisco
Unified SRST gateway.
Figure 5-5 SRST Function of Switchback
Main Site Remote Site
Cisco IP Phones, by default, wait up to 120 seconds before attempting to reestablish a
connection to a remote CUCM.
When the WAN link or connection to the primary CUCM is restored, the Cisco Unified IP
Phones reregister with their primary CUCM. 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. When switchback
is completed, call handling reverts to the primary CUCM, and SRST returns to standby
mode. The phones then return to their full functionality provided by CUCM.
SRST Timing
Typically, a phone takes three times the keepalive period to discover that its connection to
CUCM has failed. The default keepalive period, as shown in Figure 5-6, is 30 seconds.
106 Chapter 5: Examining Remote-Site Redundancy Options
Figure 5-6 SRST Timing
IP Phone
If the IP Phone has an active standby connection established with a Cisco Unified SRST
router, the fallback process takes 10 to 20 seconds after the connection with CUCM is lost.
An active standby connection to a Cisco Unified SRST router exists only if the phone has
a single CUCM in its Cisco Unified CM Group. Otherwise, the phone activates a standby
connection to its secondary CUCM.
NOTE The time it takes for an IP Phone to fall back to the SRST router can vary,
depending on the phone type. Phones such as the Cisco Unified IP Phone models 7902,
7905, and 7912 can take approximately 2.5 minutes to fall back to SRST mode.
If a Cisco Unified IP Phone has multiple CUCM systems in its Cisco Unified CM 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 connect to a CUCM. Assuming that each attempt to connect to a CUCM takes
about one minute, the Cisco Unified IP Phone in question could remain offline for three
minutes or more following a WAN link failure. You can decrease this time by setting the
MGCP Fallback Usage 107
keepalive timer to a smaller value. You can configure the keepalive timer using the CUCM
service parameter Station Keepalive Interval.
While in SRST mode, Cisco Unified IP Phones periodically attempt to reestablish a
connection with CUCM at the main site.
NOTE If you want a Cisco IP Phone to come out of SRST fallback mode faster, you can
manually reboot the phone by pressing "settings" and then **#**, or by going into the
router mode call-manager-fallback and entering reset all.
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