Cisco Unified SCCP and SIP SRST Feature Overview
Cisco Unified SCCP SRST
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Cisco Unified SCCP and SIP SRST System Administrator Guide
OL-13143-04
Information About SCCP SRST
Cisco Unified SRST provides Cisco Unified CM with fallback support for Cisco Unified IP phones that
are attached to a Cisco router on your local network. Cisco Unified SRST enables routers to provide
call-handling support for Cisco Unified IP phones when they lose connection to remote primary,
secondary, or tertiary Cisco Unified CM installations or when the WAN connection is down.
Cisco Unified CM supports Cisco Unified IP phones at remote sites attached to Cisco multiservice
routers across the WAN. Prior to Cisco Unified SRST, when the WAN connection between a router and
the Cisco Unified CM failed or when connectivity with Cisco Unified CM was lost for some reason,
Cisco Unified IP phones on the network became unusable for the duration of the failure. Cisco Unified
SRST overcomes this problem and ensures that the Cisco Unified IP phones offer continuous (although
minimal) service by providing call-handling support for Cisco Unified IP phones directly from the Cisco
Unified 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 Unified IP phones that are registered with the router. When the WAN link or connection to the
primary Cisco Unified CM is restored, call handling reverts back to the primary Cisco Unified CM.
When Cisco Unified IP phones lose contact with primary, secondary, and tertiary Cisco Unified CM,
they must establish a connection to a local Cisco Unified SRST router to sustain the call-processing
capability necessary to place and receive calls. The Cisco Unified IP phone retains the IP address of the
local Cisco Unified 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 Unified CM
accommodates a maximum of three entries. When a secondary Cisco Unified CM is not available on the
network, the local Cisco Unified SRST Router's IP address is retained as the standby connection for
Cisco Unified CM during normal operation.
Note
Cisco Unified CM fallback mode telephone service is available only to those Cisco Unified IP
phones that are supported by a Cisco Unified SRST router. Other Cisco Unified IP phones on the
network remain out of service until they re-establish a connection with their primary, secondary,
or tertiary Cisco Unified CM.
Typically, it takes three times the keepalive period for a phone to discover that its connection to Cisco
Unified CM has failed. The default keepalive period is 30 seconds. If the phone has an active standby
connection established with a Cisco Unified SRST router, the fallback process takes 10 to 20 seconds
after connection with Cisco Unified CM is lost. An active standby connection to a Cisco Unified SRST
router exists only if the phone has the location of a single Cisco Unified CM in its Unified
Communications Manager list. Otherwise, the phone activates a standby connection to its secondary
Cisco Unified CM.
Note
The time it takes for a Cisco Unified 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 Unified IP phone has multiple Cisco Unified CM in its Cisco Unified CM list, it progresses
through its list of secondary and tertiary Cisco Unified CM 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 to a
Cisco Unified CM. Assuming that each attempt to connect to a Cisco Unified CM takes about 1 minute,
the Cisco Unified IP phone in question could remain offline for 3 minutes or more following a WAN link
failure.
