Cisco SRST Configuration

Survivable Remote Site Telephony (SRST) – It is a call processing backup mechanism that allows Cisco IP phones to register to a Cisco router locally during a WAN Failure. Let’s say you are connected to your Headquarter Site A remotely via Site B using WAN. In case, the WAN links goes down, IP Phones at site B should register to the local Site B router to avoid any outages. In this case, you VoIP calls inter-clusters will not work as the WAN link is down but however calls made to the local site will work as the phones will be registered to the local router. As the Phone will register to the local site, the mode is called as SRST Fallback Mode.
Let’s do the basic configuration.
———————————————————-CUCM Configuration———————————————————-
Step 1>Configuring the SRST Reference

Go to System > SRST > Add a New SRST Reference
Name – Site B SRST
Port – 2000
IP Address : – X.X.X.X (IP Address of the Site B Router)
Click on Save

Step 2> Adding the SRST Reference in the Device Pool of the Site B Phones.

Go to System > Device Pool > Select Appropriate Device Pool which is assigned to the Phones at Site B
SRST Reference* > Select the SRST Reference created above
Click on Save, Apply and Reset (Ensure that you reset the DP after business hours)

———————————————————-SRST Configuration on Router———————————————————-
To Enable SRST on the Site B Router, you need to configure the below.

call-manager-fallback
ip source-address X.X.X.X port 2000    ——> This is the IP Address of the Site B Router
max-ephones 5
max-dn 5
system message primary SRST FALLBACK

Note:- The above configuration will only make your IP Phone fall into SRST mode. To make/receive PSTN calls, you need to configure all relevant configuration like E1/T1, dial-peers, DSPs/Transcoders etc.
During a WAN Failure, your Phone will fallback into SRST mode as shown below.

R1#
*Mar  1 01:09:37.091: %IPPHONE-6-REGISTER_NEW: ephone-1:SEP00255649DF28 IP:10.1.1.3 Socket:2 DeviceType:Phone                                                           has registered.
R1#
R1#
R1#sh ephone regi
R1#sh ephone registered
ephone-1 Mac:0025.5649.DF28 TCP socket:[2] activeLine:0 REGISTERED in SCCP ver 12 and Server in ver 5
mediaActive:0 offhook:0 ringing:0 reset:0 reset_sent:0 paging 0 debug:0 caps:6
IP:10.1.1.3 3405 Telecaster 7960  keepalive 13 max_line 6
button 1: dn 1  number 81011002  CM Fallback CH1   IDLE

Avinash Karnani

I am working in an IT company and having 10+ years of experience into Cisco IP Telephony and Contact Center. I have worked on products like CUCM, CUC, UCCX, CME/CUE, IM&P, Voice Gateways, VG224, Gatekeepers, Attendant Console, Expressway, Mediasense, Asterisk, Microsoft Teams, Zoom etc. I am not an expert but i keep exploring whenever and wherever i can and share whatever i know. You can visit my LinkedIn profile by clicking on the icon below.

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