Specifications

Overview of Cisco Unified SRST

Cisco Unified SRST Description

Cisco Unified Survivable Remote Site Telephony Version 4.0 System Administrator Guide

Cisco Unified CallManager fallback mode. When the Cisco Unified CallManager is restored, the

message goes away and full Cisco IP phone functionality is restored.

While in Cisco Unified CallManager fallback mode, Cisco IP phones periodically attempt to reestablish

a connection with Cisco Unified CallManager at the central office. Generally the default time that

Cisco IP phones wait before attempting to reestablish a connection to a remote Cisco Unified

CallManager is 120 seconds. The time can be changed in Cisco Unified CallManager; see the “Device

Pool Configuration Settings” chapter in the Cisco Unified CallManager Administration Guide. A manual

reboot can immediately reconnect Cisco Unified IP Phones to Cisco Unified CallManager.

Once a connection is reestablished with Cisco Unified CallManager, Cisco IP phones automatically

cancel their registration with the Cisco Unified SRST router. However, if a WAN link is unstable,

Cisco IP phones can bounce between Cisco Unified CallManager and Cisco Unified SRST. A

Cisco IP phone cannot reestablish a connection with the primary Cisco Unified CallManager at the

central office if it is currently engaged in an active call.

Figure 1 shows a branch office with several Cisco IP phones connected to a Cisco Unified SRST router.

The router provides connections to both a WAN link and the PSTN. The Cisco IP phones connect to their

primary Cisco Unified CallManager at the central office via this WAN link.

Figure 1 Branch Office Cisco IP Phones Connected to a Remote Central Cisco Unified

CallManager

Figure 2 shows the same branch office telephone network with the WAN connection down. In this

situation, the Cisco IP phones use the Cisco Unified SRST router as a fallback for their primary

Cisco Unified CallManager. The branch office Cisco IP phones are connected to the PSTN through the

Cisco Unified SRST router and are able to make and receive off-net calls.

IP IP IP

PSTN

network

Central

Cisco CallManager

Fax

Telephone Telephone

Cisco IP Phones

PCs

146614

Cisco Unified SRST

router