Information Technology Reference
In-Depth Information
Chapter 5
The Cisco IP telephony product range includes IOS-based devices and mechanisms, including Survivable Remote Site Telephony
(SRST), Cisco Unified Communications Manager Express (CME), and Cisco Unity Express (CUE).
Survivable Remote Site Telephony
Survivable Remote Site Telephony (SRST) is a feature that ensures that IP phones can continue to function even if they are unable to
communicate with CUCM. During a failure, Cisco IP phones register with the local SRST router, which provides call processing and
control. If Cisco IP phones detect that CUCM has come back online, they can fail back to that CUCM after a configurable interval
(connection monitor duration). Cisco IP phones do not fail back immediately to ensure that the CUCM in question is back online and
stable.
SRST is usual in deployments where Cisco IP phones at remote sites have to connect to CUCMs at a central site. In this case, SRST
can be configured at the remote site to provide redundancy in the event of a failure of connectivity to CUCM at the central site.
Basic Coniguration
The basic configuration of SRST is relatively straightforward. First, Cisco IP phones must be configured to fail over to the SRST
router (this is achieved by specifying an SRST reference within Cisco CUCM Administration), and then the SRST router to which
phones are configured to fail over must be configured for SRST.
The following example shows the minimal SRST router configuration (with SCCP):
call-manager-fallback
ip source-address 10.1.1.1 port 2000
max-ephones 20
max-dn 40
