Specifications

Overview of Cisco IOS SRST
Cisco SRST Description
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Cisco IOS Survivable Remote Site Telephony Version 3.4 System Administrator Guide
providing call-handling support for Cisco IP phones directly from the Cisco SRST router. The system
automatically detects a failure and uses Simple Network Auto Provisioning (SNAP) technology to
autoconfigure the branch office router to provide call processing for Cisco IP phones that are registered
with the router. When the WAN link or connection to the primary Cisco CallManager is restored,
call handling reverts back to the primary Cisco CallManager.
When Cisco IP phones lose contact with primary, secondary, and tertiary Cisco CallManagers, they must
establish a connection to a local Cisco SRST router to sustain the call-processing capability necessary
to place and receive calls. The Cisco IP phone retains the IP address of the local Cisco SRST router as
a default router in the Network Configuration area of the Settings menu. The Settings menu supports a
maximum of five default router entries; however, Cisco CallManager accommodates a maximum of three
entries. When a secondary Cisco CallManager is not available on the network, the local Cisco SRST
router’s IP address is retained as the standby connection for Cisco CallManager during normal operation.
Note Cisco CallManager fallback mode telephone service is available only to those Cisco IP phones that are
supported by a Cisco SRST router. Other Cisco IP phones on the network remain out of service until they
reestablish a connection with their primary, secondary, or tertiary Cisco CallManager.
Typically, it takes three times the keepalive period for a phone to discover that its connection to Cisco
CallManager has failed. The default keepalive period is 30 seconds. If the phone has an active standby
connection established with a Cisco SRST router, the fallback process takes 10 to 20 seconds after
connection with Cisco CallManager is lost. An active standby connection to a Cisco SRST router exists
only if the phone has the location of a single Cisco CallManager in its CallManager list. Otherwise, the
phone activates a standby connection to its secondary Cisco CallManager.
Note The time it takes for an IP phone to fallback to the SRST router can vary depending on the phone type.
Phones such as the Cisco 7902, Cisco 7905, and Cisco 7912 can take approximately 2.5 minutes to
fallback to SRST mode.
If a Cisco IP phone has multiple Cisco CallManagers in its CallManager list, it progresses through its
list of secondary and tertiary Cisco CallManagers before attempting to connect with its local Cisco SRST
router. Therefore, the time that passes before the Cisco IP phone eventually establishes a connection
with the Cisco SRST router increases with each attempt to contact to a Cisco CallManager. Assuming
that each attempt to connect to a Cisco CallManager takes about one minute, the Cisco IP phone in
question could remain offline for three minutes or more following a WAN link failure.
Note During a WAN connection failure, when Cisco SRST is enabled, Cisco IP phones display a message
informing you that they are operating in Cisco CallManager fallback mode. The Cisco IP Phone 7960G
and Cisco IP Phone 7940G display a “CM Fallback Service Operating” message, and the
Cisco IP Phone 7910 displays a “CM Fallback Service” message when operating in Cisco CallManager
fallback mode. When the Cisco CallManager is restored, the message goes away and full Cisco IP phone
functionality is restored.
While in Cisco CallManager fallback mode, Cisco IP phones periodically attempt to reestablish a
connection with Cisco CallManager at the central office. Generally the default time that Cisco IP phones
wait before attempting to reestablish a connection to a remote Cisco CallManager is 120 seconds. The
time can be changed in Cisco CallManager; see the “Device Pool Configuration Settings” chapter in the
Cisco CallManager Administration Guide. A manual reboot can immediately reconnect Cisco IP phones
to Cisco CallManager.