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Implementing Cisco Unified Communications Manager, Part 2 (CCNP Voice): Examining Remote-Site Redundancy Options

Chapter Description

This chapter provides an overview of the different options with remote-site redundancy in CUCM multisite installations. These different mechanisms are illustrated to help you understand how the technologies interact to deliver reliable communication services. This includes Media Gateway Control Protocol (MGCP) and Cisco Unified Survivable Remote Site Telephony (SRST).

Remote-Site Redundancy Technologies

Table 5-1 lists the capabilities of different remote-site redundancy technologies.

Table 5-1. Remote-Site Redundancy Technologies

MGCP Fallback

Cisco Unified SRST

Cisco Unified SIP SRST

Cisco Unified SIP SRST

Cisco Unified Communications Manager Express in SRST Mode

Provides redundancy for

MGCP controlled gateways

SCCP phones

SIP phones

SCCP phones

Delivered service

Fallback to Cisco IOS default technology

Basic telephony service

Basic SIP proxy service

CUCM Express

ISDN call preservation


Yes (no MGCP)

Yes (no MGCP)

Yes (no MGCP)

Analog/CAS call preservation





Maximum number of phones





To use SRST as your fallback mode on an MGCP gateway, SRST and MGCP fallback have to be configured on the same gateway.

Cisco Unified SIP SRST provides a basic set of features to SIP-based IP Phones. This set of Cisco Unified SRST basic features is also known as Cisco Unified SIP SRST. Cisco Unified SIP SRST must be enabled and configured separately on Cisco IOS routers.

Cisco Unified SIP SRST versions 3.3 and earlier provide a SIP Redirect Server function; in subsequent versions, this function acts as a back-to-back user agent (B2BUA).

CUCM Express in Cisco Unified SRST mode provides more features to a smaller maximum number of IP Phones by falling back to CUCM Express mode. The main feature enhancements include Presence, Cisco Extension Mobility, and support of local voice-mail integrations.

VoIP call preservation sustains connectivity for topologies in which signaling is managed by an entity (such as CUCM). For example, if an existing call is setup between two Cisco IP Phones with CUCM performing signaling, and CUCM goes down during the call, then the audio will stay connected since skinny has call preservation.

Call preservation is also useful when a gateway and the other endpoint (typically a Cisco Unified IP Phone) are colocated at the same site and the call agent is remote. In such a scenario, the call agent (the gateway with the remote endpoint) is more likely to experience connectivity failures.

CUCM Express version 8.0 supports a maximum of 450 IP Phones (Cisco IOS 3945E router), whereas Cisco Unified SRST version 8.0 supports up to 1500 IP Phones on the same platform. Refer to Cisco Unified Communications Manager Express 8.0, www.cisco.com/en/US/docs/voice_ip_comm/cucme/requirements/guide/cme80spc.htm, for more details.

SIP is an Internet standard discussed in many Request for Comments (RFC). If you want to supplement your knowledge of SIP, here is a good RFC to start with: www.ietf.org/rfc/rfc3261.txt.

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