Specifications

Overview of Cisco Unified SRST

Cisco Unified SRST Description

Cisco Unified Survivable Remote Site Telephony Version 4.0 System Administrator Guide

Figure 2 Branch Office Cisco IP Phones Operating in SRST Mode

H.323 Gateways and SRST

On H.323 gateways, when the WAN link fails, active calls from Cisco Unified IP Phones to the PSTN

are not maintained by default. Call preservation may work with the no h225 timeout keepalive

command, but call preservation using the no h225 timeout keepalive command is not officially

supported by Cisco Technical Support.

Under default configuration, the H.323 gateway maintains a keepalive signal with Cisco Unified

CallManager and terminates H.323-to-PSTN calls if the keepalive signal fails, for example if the WAN

link fails. To disable this behavior and help preserve existing calls from local IP phones, you can use the

no h225 timeout keepalive command. Disabling the keepalive mechanism only affects calls that will be

torn down as a result of the loss of the H.225 keepalive signal. For information regarding disconnecting

a call when an inactive condition is detected. see the Media Inactive Call Detection document.

MGCP Gateways and SRST

MGCP fallback is a different feature than SRST and, when configured as an individual feature, can be

used by a PSTN gateway. To use SRST as your fallback mode on an MGCP gateway, SRST and MGCP

fallback must both be configured on the same gateway. MGCP and SRST have had the capability to be

configured on the same gateway since Cisco IOS Release 12.2(11)T.

To make outbound calls while in SRST mode on your MGCP gateway, two fallback commands must be

configured on the MGCP gateway. These two commands allow SRST to assume control over the voice

port and over call processing on the MGCP gateway. With Cisco IOS releases prior to 12.3(14)T, the two

commands are the ccm-manager fallback-mgcp and call application alternate commands. With

Cisco IOS releases after 12.3(14)T, the ccm-manager fallback-mgcp and service commands must be

configured. A complete configuration for these commands is shown in the section “Enabling SRST on

an MGCP Gateway” section on page 40.

IP IP IP

PSTN

network

WAN

disconnected

Central

Cisco CallManager

Cisco Unified SRST

router

Fax

Telephone Telephone

Cisco IP phones

PCs

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