Specifications

Cisco IOS Survivable Remote Site Telephony Version 3.4 System Administrator Guide

Overview of Cisco IOS SRST

This chapter describes Cisco Survivable Remote Site Telephony (SRST) and what it does. It also

includes information about Cisco IP phone, platform, and Cisco CallManager version support;

specifications; features; restrictions; and where to find additional reference documents.

Note For the most up-to-date information about Cisco IP phone support, the maximum number of Cisco IP

phones, maximum DNs or virtual voice ports, and memory requirements for Cisco SRST, see the Cisco

Survivable Remote Site Telephony (SRST) 3.4 Specifications for Cisco IOS Release 12.4(4)T at the

following URL:

http://www.cisco.com/univercd/cc/td/doc/product/voice/srst/srst34/srs34spc.htm

Contents

• Cisco SRST Description, page 19

• Support for Cisco IP Phones, Platforms, Cisco CallManager, Signals, Languages, and Switches,

page 23

• Prerequisites for Configuring Cisco SRST, page 26

• Restrictions for Configuring Cisco SRST, page 29

• Where to Go Next, page 30

• Additional References, page 30

Cisco SRST Description

Cisco SRST provides Cisco CallManager with fallback support for Cisco IP phones that are attached to

a Cisco router on your local network. Cisco SRST enables routers to provide call-handling support for

Cisco IP phones when they lose connection to remote primary, secondary, or tertiary Cisco CallManager

installations or when the WAN connection is down.

Cisco CallManager supports Cisco IP phones at remote sites attached to Cisco multiservice routers

across the WAN. Prior to Cisco SRST, when the WAN connection between a router and the

Cisco CallManager failed or when connectivity with Cisco CallManager was lost for some reason,

Cisco IP phones on the network became unusable for the duration of the failure. Cisco SRST overcomes

this problem and ensures that the Cisco IP phones offer continuous (although minimal) service by