This feature enables
routers to provide call-handling support for Cisco Unified IP phones if they
lose connection to remote primary, secondary, or tertiary
Cisco Unified Communications Manager installations or if the WAN connection is
down. When Cisco Unified SRST functionality is provided by Cisco Unified CME,
provisioning of phones is automatic and most Cisco Unified CME 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 protocol.
The benefit is that Cisco Unified Communications Manager users will gain access
to more features during fallback without any additional licensing costs.
This feature offers
a limited telephony feature set during fallback mode. Customers who require the
following features should continue to use Cisco Unified SRST, because these
features are not supported with SRST fallback support using Cisco Unified CME.
More than 240
phones during fallback service
Cisco VG 248
Analog Phone Gateway support
fallback during SRST fallback service
configuration for SRST fallback service
Cisco Unified Communications Manager supports Cisco Unified IP
phones at remote sites attached to Cisco Integrated Services Routers across the
WAN. This new feature combines the many features available in Cisco Unified CME
with the ability to automatically detect IP phone configurations that is
available in Cisco Unified SRST to provide seamless call handling when
communication with the Cisco Unified Communications Manager is interrupted.
When the system
automatically detects a failure, Cisco Unified SRST uses Simple Network Auto
Provisioning (SNAP) technology to auto-configure a branch office router to
provide call processing for the Cisco Unified IP phones that are registered
with the router. When the WAN link or connection to the primary
Cisco Unified Communications Manager is restored, call handling returns to the
primary Cisco Unified Communications Manager.
A limited number of
phone features are automatically detected at the time that call processing
falls back to Cisco Unified CME in SRST Fallback Mode, and an advantage of SRST
fallback support using Cisco Unified CME is that you can choose to prebuild a
Cisco Unified CME configuration that contains a number of extensions
(ephone-dns) with additional features that you want them to have for some or
all of your extensions. The configurations will contain ephone-dn
configurations but will not identify which phones (which MAC addresses) will be
associated with which ephone-dns (extension numbers).
By copying and
pasting a prebuilt configuration onto Cisco Unified CME routers at several
locations, you can use the same overall configuration for sites that are
identically laid out. For example, if you have a number of retail stores, each
with five to ten checkout registers, you can use the same overall configuration
in each store. You might use a range of extensions from 1101 to 1110. Stores
with fewer than ten registers will simply not use some of the ephone-dn entries
you provide in the configuration. Stores with more extensions than you have
prebuilt will use the auto-provisioning feature to populate their extra phones.
The only configuration variations from store to store will be the specific MAC
addresses of the individual phones, which are added to the configurations at
the time of fallback.
When a phone
registers for SRST service with a Cisco Unified CME router and the router
discovers that the phone was configured with a specific extension number, the
router searches for an existing prebuilt ephone-dn with that extension number
and then assigns that ephone-dn number to the phone. If there is no prebuilt
ephone-dn with that extension number, the Cisco Unified CME system
automatically creates one. In this way, extensions without prebuilt
configurations are automatically populated with extension numbers and features
as the numbers and features are “learned” by the Cisco Unified CME router in
SRST mode when the phone registers to the router after a WAN link fails.
The SRST fallback
support using Cisco Unified CME feature is able to interrogate phones to learn
their MAC addresses and the extension-to-ephone relationships associated with
each phone. This information is used to dynamically create and execute the
Cisco Unified CME
command for each phone and automatically provision each phone with the
extensions and features you want it to have.
sequence describes how Cisco Unified CME provides SRST services for
Cisco Unified Communications Manager phones when they lose connectivity with
the Cisco Unified Communications Manager and fall back to the Cisco Unified CME
router in SRST mode:
configured as usual in Cisco Unified Communications Manager.
The IP address
of the Cisco Unified CME router is registered as the SRST reference on the
Cisco Unified Communications Manager device pool.
SRST mode is
enabled on the Cisco Unified CME router.
Ephone-dns and features are prebuilt on the Cisco Unified CME router.
Phones that are enabled for fallback register to the default
Cisco Unified CME router that has SRST mode enabled. Each display-enabled IP
phone displays the message that has been defined using the
system message command under
telephony-service configuration mode. By default, this message is
“Cisco Unified CME.”
While the fallback phones are registering, the router in SRST mode
initiates an interrogation of the phones in order to learn their phone and
extension configurations. The following information is acquired or “learned” by
The option defined with the
srst mode auto-provision command determines
whether Cisco Unified CME adds the learned phone and extension information to
its running configuration. If the information is added, it appears in the
output when you use the
show running-config command and is saved to
NVRAM when you use the
srst mode auto-provision none command
to enable the Cisco Unified CME router to provide SRST fallback services for
Cisco Unified Communications Manager.
If you use the
srst mode auto-provision
srst mode auto-provision
all commands, the Cisco Unified CME
router includes the phone configuration it learns from
Cisco Unified Communications Manager in its running configuration. If you then
save the configuration, the fallback phones are treated as locally configured
phones on the Cisco Unified CME-SRST router which could adversely impact the
fallback behavior of those phones.
While in fallback mode, Cisco Unified IP phones periodically attempt
to reestablish a connection with Cisco Unified Communications Manager every 120
seconds (default). To manually reestablish a connection to
Cisco Unified Communications Manager you can reboot the Cisco Unified IP phone.
- When a connection is
reestablished with Cisco Unified Communications Manager, Cisco Unified IP
phones automatically cancel their registration with the Cisco Unified CME
router in SRST mode. However, if a WAN link is unstable, Cisco Unified IP
phones can bounce between Cisco Unified Communications Manager and the
Cisco Unified CME router in SRST mode.
An IP phone
connected to the Cisco Unified CME-SRST router over a WAN reconnects itself to
Cisco Unified Communications Manager as soon as it can establish a connection
to Cisco Unified Communications Manager over the WAN link. However, if the WAN
link is unstable, the IP phone switches back and forth between
Cisco Unified CME-SRST and Cisco Unified Communications Manager, causing
temporary loss of phone service (no dial tone). These reconnect attempts, known
as WAN link flapping issues, continue until the IP phone successfully
reconnects itself back to Cisco Unified Communications Manager.
WAN link disruptions
can be classified into two types: infrequent random outages that occur on an
otherwise stable WAN, and sporadic, frequent disruptions that last a few
To resolve WAN-link
flapping issues between Cisco Unified Communications Manager and SRST,
Cisco Unified Communications Manager provides an enterprise parameter and a
setting in the Device Pool Configuration window called Connection Monitor
Duration. (Depending on system requirements, the administrator decides which
parameter to use.) The value of the parameter is delivered to the IP phone in
the XML configuration file.
Use the enterprise parameter to change the connection duration monitor value for all IP phones in the Cisco Unified Communications Manager cluster. The default for the enterprise parameter is 120 seconds.
Use the Device Pool Configuration window to change the connection duration monitor value for all IP phones in a specific device pool.
A Cisco Unified IP phone will not reestablish a connection with the primary Cisco Unified Communications Manager at the central office if it is engaged in an active call.
After the First
can be set up, such as ephone hunt groups, which can contain learned extensions
and prebuilt extensions. The complete core set of Cisco Unified CME phone
features is available to the IP phones and extensions, whether they are learned
Figure 51-1 shows a branch office with
several Cisco Unified IP phones connected to a Cisco Unified CME router in SRST
fallback mode. The router provides connections to both a WAN link and the PSTN.
The Cisco Unified IP phones connect to their primary
Cisco Unified Communications Manager at the central office via this WAN link.
Cisco Unified CME provides SRST services for the phones when connectivity over
the WAN link is interrupted.
Figure 1. SRST Fallback
Support using Cisco Unified CME