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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:


  1. Define the SRST reference for phones in CUCM administration
  2. 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.
  3. Enable and configure the MGCP fallback and Cisco Unified SRST features on the IOS gateways.
  4. 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


MGCP fallback-1.bmp


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* :

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


RemoteSite(config)# call-manager-fallback

! -- call-manager-fallback enables SRST on the IOS router


RemoteSite(config-cm-fallback)# ip source-address port 2000

! -- defining the ip source-address. This ip address will be supplied as SRST reference IP address.


RemoteSite(config-cm-fallback)# max-ephones 3 dual-line

! - defining the SRST phone max-ephones


RemoteSite(config-cm-fallback)# max-dn 6

! - defining the SRST phone max-dn


RemoteSite(config-cm-fallback)# limit-dn 7960 2

! - limit dn to optimize the performance of the system


RemoteSite(config-cm-fallback)# keepalive 20


RemoteSite(config-cm-fallback)# end





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)# application


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.


Dial Peer configuration for SRST Dial plan


RemoteSite# configure terminal


RemoteSite(config)# dial-peer voice tag pots


RemoteSite(config-dial-peer) destination-pattern [+]string[T]


Remotesite(config-dial-peer) port slot-number/port



Sample configuration for SRST dialplan



dial-peer voice 1 pots

description PSTN-emergency dial 9 first

destination-pattern 9911

port 0/1/0:23

forward-digits 3



dial-peer voice 2 pots

description PSTN-emergency

destination-pattern 911

port 0/1/0:23

forward-digits all



dial-peer voice 3 pots

description PSTN-Local services

destination-pattern 9[2-8]11

port 0/1/0:23

forward-digits 3



dial-peer voice 4 pots

description PSTN-Long distance Dialing

destination-pattern 91[2-9]..[2-9]......

port 0/1/0:23

forward-digits 11



dial-peer voice 5 pots

description PSTN-International Dialing

destination-pattern 9011T

port 0/1/0:23

prefix 011



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