What Is Cisco SRST? (Survivable Remote Site Telephony)
SRST (Survivable Remote Site Telephony) is a Cisco feature that allows branch office phones to continue functioning when the central CUCM server becomes unreachable due to WAN failure. This ensures uninterrupted calling capability for remote sites.
Why SRST is Needed
In multi-site Cisco deployments, phones depend on CUCM for call control. If WAN goes down, phones lose service. SRST provides backup intelligence inside the router to keep phones working.
How SRST Works
- CUCM normally controls phones
- If WAN link fails → SRST router takes over
- Phones register automatically with SRST
- Basic features remain available
Features Available in SRST Mode
- Internal extension calling
- External PSTN calling (via gateway)
- Basic hold, transfer, forward
- Limited directory
Example SRST Configuration
call-manager-fallback max-ephones 40 max-dn 80 ip source-address 192.168.10.1 port 2000
SRST vs CME
| Feature | SRST | CME |
|---|---|---|
| Purpose | Backup for CUCM | Standalone PBX |
| Function | Emergency call control | Primary call control |
Benefits of SRST
- Ensures telephony continuity
- Reduces downtime
- Critical for branch offices
Conclusion
SRST is a powerful failover mechanism ensuring remote sites continue calling even if central servers go offline. Every enterprise multi-site VoIP deployment should enable SRST to maintain business continuity and emergency calling functionality.
