Hi Abishek,

There are two flavors of SRST (as shown below). Here is the feature comparison;

SRST Fallback

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.

Comparison of Cisco Unified SRST and

Cisco Unified CME in SRST Fallback Mode

Cisco Unified CME in SRST Fallback Mode

• First supported with Cisco Unified CME 4.0: Cisco IOS Software 12.4(9)T

• IP phones re-home to Cisco Unified CME if Cisco Unified Communications Manager fails. CME in SRST allows IP phones to access some advanced Cisco Unified CME telephony features not supported in traditional SRST

• Support for up to 240 phones

• No support for Cisco VG248 48-Port Analog Phone Gateway registration during fallback

• Lack of support for alias command

• Support for Cisco Unity® unified messaging at remote sites (Distributed Exchange or Domino)

• Support for features such as Pickup Groups, Hunt Groups, Basic Automatic Call Distributor (BACD), Call Park, softkey templates, and paging

• Support for Cisco IP Communicator 2.0 with Cisco Unified Video Advantage 2.0 on same computer

• No support for secure voice in SRST mode

• More complex configuration required

• Support for digital signal processor (DSP)-based hardware conferencing

• E-911 support with per-phone emergency response location (ERL) assignment for IP phones (Cisco Unified CME 4.1 only)

Cisco Unified SRST

• Supported since Cisco Unified SRST 2.0 with Cisco IOS Software 12.2(8)T5

• IP phones re-home to SRST router if Cisco Unified Communications Manager fails. SRST allows IP phones to have basic telephony features

• Support for up to 720 phones

• Support for Cisco VG248 registration during fallback

• Support for alias command

• Lack of support for features such as Pickup Groups, Hunt Groups, Call Park, and BACD

• No support for Cisco IP Communicator 2.0 with Cisco Unified Video Advantage 2.0

• Support for secure voice during SRST fallback

• Simple, one-time configuration for SRST fallback service

• No per-phone emergency response location (ERL) assignment for SCCP Phones (E911 is a new feature supported in SRST 4.1)

http://www.cisco.com/en/US/prod/collateral/voicesw/ps6788/vcallcon/ps2169/prod_qas0900aecd8028d113.html

Cisco Unified SRST System Administrator Guide

http://www.cisco.com/en/US/docs/voice_ip_comm/cusrst/admin/srst/configuration/guide/srstsa.html

Hope this helps!

Rob