Upon completing this chapter, you will be able to describe the mechanisms for providing call survivability and device failover at remote sites, including the functions, operation, and limitations of each mechanism. You will be able to meet these objectives:
- Describe remote-site redundancy options and compare their characteristics
- Describe how Cisco Unified SRST works
- Describe how MGCP fallback works
- Describe SRST versions, their protocol support and features, and the required Cisco IOS Software release
- Describe dial plan requirements for MGCP fallback and SRST with the option of using CUCM Express
Remote-Site Redundancy Overview
Two different technologies are used to provide remote-site redundancy for small and medium remote sites in a Cisco Unified Communications Manager (CUCM) environment. Survivable Remote Site Telephony (SRST) and Media Gateway Control Protocol (MGCP) gateway fallback are the key components of delivering fail-safe communication services, as shown in Figure 5-1.
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Figure 5-1 SRST and MGCP Fallback
CUCM supports Cisco Unified IP Phones at remote sites that are attached to Cisco multiservice routers across the WAN. Before Cisco Unified SRST, when the WAN connection between a router and the CUCM failed, or when connectivity with the CUCM was lost for any reason, Cisco Unified IP Phones on the network became unusable for the duration of the failure. The reason is that Cisco IP Phones demand Skinny Client Control Protocol (SCCP) or Session Initiation Protocol (SIP) connectivity to a call-processing agent such as CUCM, and in the absence of signaling connectivity, the phones become fully unusable.
Cisco Unified SRST overcomes this problem and ensures that Cisco Unified IP Phones offer continuous, although reduced, service by providing call-handling support for Cisco Unified IP Phones directly from the Cisco Unified SRST router. The system automatically detects a failure and uses Simple Network-Enabled Auto-Provision (SNAP) technology to autoconfigure the branch office router to provide call processing for Cisco Unified IP Phones that are registered with the router. When the WAN link or connection to the primary CUCM subscriber is restored, call handling reverts to the primary CUCM.
MGCP gateway fallback is a mechanism that allows a Cisco IOS router to continue providing voice gateway functions, even when the MGCP call agent is not in control of the media gateway. These voice gateway functions are implemented through a fallback mechanism that activates the so-called default technology application. The gateway then works in the same way as a standalone H.323 or SIP gateway by using its configured dial peers.