In this post, I am going to configure Survivable Remote Site Telephony (SRST) between CUCM (site A) and a cisco router 2911 (Site B) for a call processing backup mechanism that allows IP phones to register when a WAN link is disconnected.
Assume that the WAN link goes down, Cisco IP phones at site B should register to the local router (site B) to avoid any disruptions. According to the below diagram, until the WAN link is up, cisco IP phones in site B are registered to the CUCM at site A, whenever the WAN link is disconnected cisco IP phones are reregistered to the router B and will be able to make calls in the local site. As the phone will register to the local site, the model is called SRST Fallback mode.
Let’s do the CUCM Configuration side
Step 1: Log into your CUCM then go to System > SRST
Step 2: Click on Add New
Step 3: Enter a Name, Port (2000 is the Default), IP address, SIP Port , Click Save
Step 4: Go to System > Device Pool
Step 5: Add a device pool related to the site B
Step 6: Locate SRST in the list of settings, select the reference that you just added to the system
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut elit tellus, luctus nec ullamcorper mattis, pulvinar dapibus leo.Step 7: Go to Device > Phone, Find the device which you want to enable SRST mode then select the device pool that you created in step 5
SRST Configuration on Router B (2911)
Step 1:RouterB(config)#Call-manager-fallback (To active SRST)
Step 2: Define the IP address and port to which the SRST service binds (Site B Router)
RouterB(config-cm-fallback)# ip source-address 172.16.0.1 port 2000
Step 3: Define the maximum number of directory numbers (DN) to support
RouterB(config-cm-fallback)#max-dn 10
Step 4: Define the maximum number of IP phones to support
RouterB(config-cm-fallback)# Max-ephones 10
Step 5: Define the phone keep alive interval
RouterB(config-cm-fallback)# keepalive 30
Basic SRST configuration is done here, but if you want to have a connection to make external calls in SRST mode, I would recommend installing a FXO module
RouterB(config)# dial-peer voice 9 pots
Destination-pattern 9T
Port 0/0/0
