This document describes how to configure Cisco Unified Survivable Remote Site Telephony (SRST) on Cisco IOS routers to provide redundancy to Cisco Skinny Client Protocol (SCCP) Phones. It also describes how to configure the Media Gateway Control Protocol (MGCP) gateway fallback feature.
Basic Cisco Unified SRST Usage
Cisco Unified SRST provides CUCM with fallback support for Cisco Unified IP Phones that are attached to a Cisco router on a local network. Cisco Unified SRST enables routers to provide basic call-handling support for Cisco Unified IP Phones when they lose connection to remote primary, secondary and teritiary CUCM servers or when the WAN connection to the main site is down.
MGCP Fallback Usage
A Public Switched Telephone Network (PSTN) gateway can use MGCP gateway fallback configured as an individual feature if H.323 or SIP is configured as a backup service. SRST and MGCP fallback must be configured on the same gateway with Cisco IOS software release 12.2(11)T or later if this single gateway will provide SRST fallback service to phones and MGCP gateway fallback.
MGCP Fallback and SRST Configuration
The MGCP gateway fallback feature is activated and configured on the Cisco IOS router. Note that Cisco Unified SRST must be configured within CUCM and within the Cisco IOS Router.
When configuring MGCP Fallback and Cisco Unified SRST, you must follow these steps at different locations:
Define the SRST reference for phones in CUCM administration
Configure the Call Forward Unregistered (CFUR) feature and set the CFUR destination of lines on remote-site phones to the correct public switched telephone network (PSTN) number in CUCM administration to enable reachable remote sites in SRST mode.
Enable and configure the MGCP fallback and Cisco Unified SRST features on the IOS gateways.
Implement a simplified SRST dial plan on the remote-site-gateways to ensure connectivity for remote-site phones in SRST mode.
Figure shows the MGCP fallback and Cisco Unified SRST
1. Cisco Unified SRST Configuration in CUCM
The SRST feature in Cisco Unified Communication Manager (CUCM) provides IP Phones with the information needed to find the relative gateway to register with when they lose contact with CUCM servers.
SRST Reference Definition: SRST reference comprises the gateway which can provide limited CUCM functionality when all other CUCM Servers for IP Phones are unreachable.
Follow the procedure to configure SRST in CUCM at the Main Site.
SRST Configuration in CUCM Administration Page
From Cisco Unified CMAdministration, choose System > SRST > Add New.
SRST Reference Information
Name* : SRST-Remote1
Port* : 2000
IP Address* : 126.96.36.199
SIP Network/IP Address :
SIP Port* : 5060
SRST certificate Provider port* : 2445
Is SRST secure ?
CUCM Device Pool Configuration
The SRST reference as shown above is assigned to IP Phones using Device Pools.
CUCM Device Pool configuration in CUCM Administration Page
From Cisco Unified CMAdministration, choose System >Device Pool > Add New.
Administrators select the configured SRST reference from the drop-down menu in the device pool configuration.
Roaming Sensitive Settings
Date/Time Group* : CMLocal
Region* : Default
Media Resource Group list : None
Location : None
Network Locale : None
SRST reference* : SRST-Remote1
Connection Monitor Duration : None
Physical Location : None
Device Mobility Group : None
2. SRST Configuration on the Cisco IOS Gateway
Follow these steps to configure Cisco Unified SRST on a Cisco IOS gateway at the Remote Site
Step1: Enter call-manager-fallback configuration mode to activate SRST
Step2: Define the IP address and port to which the SRST service binds
Step3: Define the maximum number of DNs to support
Step4: Define the maximum number of IP Phones to support
Step5: Define the maximum number of numbers allowed per phone type
Step6: Define the phone keepalive interval (optional)
Note: When Cisco Unified SRST is enabled, Cisco IP Phones in call-manager-fallback configuration mode do not have to be reconfigured because phones retain the same configuration that was used with CUCM.
Cisco Unified SRST Configuration Example
RemoteSite# configure terminal
! -- call-manager-fallback enables SRST on the IOS router
RemoteSite(config-cm-fallback)# ip source-address 188.8.131.52 port 2000
! -- defining the ip source-address. This ip address will be supplied as SRST reference IP address.
! - limit dn to optimize the performance of the system
RemoteSite(config-cm-fallback)# keepalive 20
3. MGCP- Gateway-Fallback Configuration on the Cisco IOS Gateway
To use SRST on an MGCP Gateway, you must configure SRST and MGCP gateway fallback on the same gateway.
Follow these steps and configure the MGCP fallback on the Cisco IOS gateway at the Remote Site.
Step1: Activate MGCP gateway fallback.
Step2: Define the service to fall back to.
To enable outbound calls while in SRST mode on an MGCP gateway, you must configure two fallback commands on the MGCP gateway. These two commands allow SRST to assume control over the voice port and over call processing on the MGCP gateway.
MGCP Fallback Configuration Example
RemoteSite# configure terminal
RemoteSite(config)# call-manager fallback-mgcp
RemoteSite(config - app)# global
RemoteSite(config - app -global)# service alternate default
RemoteSite(config - app -global)# end
4. Cisco Unified SRST Dial Peer Configuration
Follow these steps to configure SRST Dial-peer on the Cisco IOS gateway at the Remote Site.
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