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 supports Cisco IP phones at remote sites attached to Cisco multiservice

routers across the WAN. Prior to Cisco Unified SRST, when the WAN connection between a router and

the Cisco Unified CallManager failed or when connectivity with Cisco Unified CallManager was lost for

some reason, Cisco IP phones on the network became unusable for the duration of the failure.

Cisco Unified SRST overcomes this problem and ensures that the Cisco IP phones offer continuous

(although minimal) service by providing call-handling support for Cisco IP phones directly from the

Cisco Unified SRST router. The system automatically detects a failure and uses Simple Network Auto

Provisioning (SNAP) technology to autoconfigure the branch office router to provide call processing for

Cisco IP phones that are registered with the router. When the WAN link or connection to the primary

Cisco Unified CallManager is restored, call handling reverts back to the primary Cisco Unified

CallManager.

When Cisco IP phones lose contact with primary, secondary, and tertiary Cisco Unified CallManagers,

they must establish a connection to a local Cisco Unified SRST router to sustain the call-processing

capability necessary to place and receive calls. The Cisco IP phone retains the IP address of the local

Cisco Unified SRST router as a default router in the Network Configuration area of the Settings menu.

The Settings menu supports a maximum of five default router entries; however, Cisco Unified

CallManager accommodates a maximum of three entries. When a secondary Cisco Unified CallManager

is not available on the network, the local Cisco Unified SRST router’s IP address is retained as the

standby connection for Cisco Unified CallManager during normal operation.

Note Cisco Unified CallManager fallback mode telephone service is available only to those Cisco IP phones

that are supported by a Cisco Unified SRST router. Other Cisco IP phones on the network remain out of

service until they reestablish a connection with their primary, secondary, or tertiary Cisco Unified

CallManager.

Typically, it takes three times the keepalive period for a phone to discover that its connection to

Cisco Unified CallManager has failed. The default keepalive period is 30 seconds. If the phone has an

active standby connection established with a Cisco Unified SRST router, the fallback process takes 10

to 20 seconds after connection with Cisco Unified CallManager is lost. An active standby connection to

a Cisco Unified SRST router exists only if the phone has the location of a single Cisco Unified

CallManager in its Unified CallManager list. Otherwise, the phone activates a standby connection to its

secondary Cisco Unified CallManager.

Note The time it takes for an IP phone to fallback to the SRST router can vary depending on the phone type.

Phones such as the Cisco 7902, Cisco 7905, and Cisco 7912 can take approximately 2.5 minutes to

fallback to SRST mode.

If a Cisco IP phone has multiple Cisco Unified CallManagers in its CallManager list, it progresses

through its list of secondary and tertiary Cisco Unified CallManagers before attempting to connect with

its local Cisco Unified SRST router. Therefore, the time that passes before the Cisco IP phone eventually

establishes a connection with the Cisco Unified SRST router increases with each attempt to contact to a

Cisco Unified CallManager. Assuming that each attempt to connect to a Cisco Unified CallManager

takes about one minute, the Cisco IP phone in question could remain offline for three minutes or more

following a WAN link failure.

Note During a WAN connection failure, when Cisco Unified SRST is enabled, Cisco IP phones display a

message informing you that they are operating in Cisco Unified CallManager fallback mode. The

Cisco IP Phone 7960G and Cisco IP Phone 7940G display a “CM Fallback Service Operating” message,

and the Cisco IP Phone 7910 displays a “CM Fallback Service” message when operating in