What is SRST? Difference between SRST and E-SRST

Survivable Remote Site Telephony (SRST)


Normally Cisco IP Phones registers to CUCM whether you use Multi-site with Centralized/Distributed Call Processing Model, Clustering over IP WAN or Single Site Deployment. In the event of WAN failure or the primary, secondary and tertiary CUCM server goes down, the remote site end points will not be able to reach Cisco Call Manager and the IP Phone’s registration will be lost and call processing/call control also will not work. In such cases, Survivable Remote Site Telephony (SRST) can provide call setup and call teardown services for the sites that do not have a local CUCM server. SRST will offer basic telephony features to the endpoints and they can make PSTN calls and VoIP calls within the local site (no calls will be routed over WAN as the WAN link is down or may the CUCMs are not accessible ). SRST is an IOS router feature.

Survivable Remote Site Telephony Hardware Requirements

Cisco Router Maximum Cisco Unified IP Phones
1751-V/1761-V 24
1760 24
261xXM/262xXM 36
2650XM/2651XM 48
2691 72
2801 24
2811 36
2821 48
2851 96
3725 144
3825 336
3845 720
6500 CMM 480


There are two types of SRST supported by Cisco IOS gateways :

  • SRST (call manager fallback, traditional) – This offers basic telephony features.
  • Cisco Unified CME-based SRST or enhanced SRST (E-SRST) – E-SRST provides an enhanced feature set for the endpoints. E-SRST builds on top of SRST by automatically synchronizing user, phone, hunt group, partitions, and so on from CUCM to Cisco Unified CME. Any change on CUCM is propagated to the branch sites without admin intervention. So when the WAN is down, the phone resembles the last changes performed on CUCM. When in E-SRST mode, Cisco Unified CME syncs automatically with CUCM to get updates as and when they happen on CUCM.

The following table shows some additional difference between SRST and E-SRST.

Cisco Unified E-SRST Cisco Unified SRST
450 Total phones are supported 1500 Total phones are supported
Cisco VG248 48-port analog phone gateway is not supported for fallback in E-SRST Cisco VG248 48-port analog phone gateway is supported for fallback in SRST mode
Alias command is not supported Alias command is supported
Enhanced features such as call park, call pick, ephone-template, video survivability, BLF, shared-line are supported with E-SRST Only basic features are supported with SRST. 
Class of restriction is a supported feature in E-SRST Class of restriction is not supported in SRST
SRST Manager supports provisioning of E-SRST SRST Manager supports provisioning of SRST from SRST Manager 9.0
N/A Supports Cisco Hosted Collaboration Solution (HCS)


Hope this helps!

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