Cisco Unified SCCP and SIP SRST Feature Overview
Cisco Unified SCCP SRST
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Cisco Unified SCCP and SIP SRST System Administrator Guide
OL-13143-04
Note
During a WAN connection failure, when Cisco Unified SRST is enabled, Cisco Unified IP
phones display a message informing you that they are operating in Cisco Unified CM fallback
mode. For example, the Cisco Unified IP Phone 7960G and Cisco Unified IP Phone 7940G
display a "CM Fallback Service Operating" message, and the Cisco Unified IP Phone 7910
displays a "CM Fallback Service" message when operating in Cisco Unified CM fallback mode.
When the Cisco Unified CM is restored, the message goes away and full Cisco Unified IP phone
functionality is restored.
While in Cisco Unified CM fallback mode, Cisco Unified IP phones periodically attempt to re-establish
a connection with Cisco Unified CM at the central office. Generally, the default time that Cisco Unified
IP phones wait before attempting to re-establish a connection to a remote Cisco Unified CM is 120
seconds. The time can be changed in Cisco Unified CM; see the "Device Pool Configuration Settings"
chapter in the appropriate Cisco Unified CM Administration Guide. A manual reboot can immediately
reconnect Cisco Unified IP phones to Cisco Unified CM.
When a connection is re-established with Cisco Unified CM, Cisco Unified IP phones automatically
cancel their registration with the Cisco Unified SRST Router. However, if a WAN link is unstable, Cisco
Unified IP phones can bounce between Cisco Unified CM and Cisco Unified SRST. A Cisco Unified IP
phone cannot re-establish a connection with the primary Cisco Unified CM at the central office if it is
currently engaged in an active call.
Cisco Unified SRST supports the following call combinations:
•
SCCP phone to SCCP phone
•
SCCP phone to PSTN/router voice-port
•
SCCP phone to WAN VoIP using SIP or H.323
•
SIP phone to SIP phone
•
SIP phone to PSTN / router voice-port
•
SIP phone to Skinny Client Control Protocol (SCCP) phone
•
SIP phone to WAN VoIP using SIP
Figure 1
shows a branch office with several Cisco Unified IP phones connected to a Cisco Unified SRST
router. The router provides connections to both a WAN link and the PSTN. Typically, the
Cisco Unified IP phones connect to their primary Cisco Unified Communications Manager at the central
office via the WAN link. When the WAN connection is down, the Cisco Unified IP phones use the
Cisco Unified SRST router as a fallback for their primary Cisco Unified Communications Manager. The
branch office Cisco Unified IP phones are connected to the PSTN through the Cisco Unified SRST
router and are able to make and receive off-net calls.
