Specifications

Overview of Cisco IOS SRST

Cisco SRST Description

Cisco IOS Survivable Remote Site Telephony Version 3.4 System Administrator Guide

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

registration with the Cisco SRST router. However, if a WAN link is unstable, Cisco IP phones can bounce

between Cisco CallManager and Cisco SRST. A Cisco IP phone cannot reestablish a connection with the

primary Cisco 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 SRST router. The

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

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

Figure 1 Branch Office Cisco IP Phones Connected to a Remote Central Cisco 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 SRST router as a fallback for their primary Cisco

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

62141

Cisco SRST

router