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Figure 6-1 shows the topology relating to configuring Cisco Unified SRST and MGCP
gateway fallback on Cisco IOS routers.
Figure 6-1 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.
Configuration Requirements for MGCP Fallback and Cisco Unified SRST
When configuring MGCP fallback and Cisco Unified SRST, you must follow these steps at
different locations:
• Define the SRST references 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.
Cisco Unified SRST Configuration in CUCM 125
Cisco Unified SRST Configuration in CUCM
The SRST feature in CUCM provides IP Phones with the information needed to find the
relative gateway to register with when they lose contact with CUCM servers.
An SRST reference must first be defined. This reference contains information about IP
addresses and ports of SRST gateways for SCCP and session initiation protocol (SIP)
phones. Because the SRST functions are different for SIP and SCCP, the addresses and
ports are also different.
Secondly, provide a group of phones with this information by assigning the SRST reference
to a proper device pool, which is then assigned to the phones.
SRST Reference Definition
An SRST reference, as shown in Figure 6-2, comprises the gateway, which can provide
limited CUCM functionality when all other CUCM servers for IP Phones are unreachable.
Choose Cisco Unified Communications Manager Administration > System > SRST.
Figure 6-2 SRST Reference Definition in CUCM
i— SRST Reference Information
* Name-
PortJ
SRST-Remotel
2 0 0 0
IP Address*
SIP Network/IP Address
SIP Port*
SRST Certificate Provider
Port*
• Is SRST Secure?
172.42.2. l|
5060
2445
SRST references determine which gateways IP Phones will search when they attempt to
complete a call if the CUCM is unavailable.
Administrators must configure CUCM with a unique SRST reference name that specifies
the IP address of the Cisco Unified SRST gateway. The default TCP port number 2000
normally is used.
The SIP network and IP address apply to SIP SRST. If SIP SRST is used, the IP address
and port that are used by the SIP protocol of the Cisco Unified SRST gateway have to be
specified; the default port number is 5060. The configured address and port will be used by
SIP phones to register with the SIP SRST gateway.
126 Chapter 6: Implementing Cisco Unified SRST and MGCP Fallback
CUCM Device Pool
The SRST reference, as shown in Figure 6-3, is assigned to IP Phones using device pools.
Choose Cisco Unified Communications Manager Administration > System > Device
Pool.
Figure 6-3 Device Pool in CUCM
I— Roaming Sensitive Settings
Date/Time Group* CMLocal
Region* Default
Media Resource Group List *~ None >
Location c None >
Network Locale < None >
SRST Reference*
Connection Monitor D u r a t i o n * * * "
Physical Location < None >
Device Mobility Group < None >
Administrators select the configured SRST reference from the drop-down menu in the
device pool configuration.
NOTE If devices are associated with this SRST reference, a message appears, saying
that devices need to be reset for the update to take effect.