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CCNP Voice Quick Reference: Centralized Call Processing Redundancy

Chapter Description

This chapter reviews the mechanisms for providing call survivability and device failover in remote sites. This includes the configuration of Cisco IOS routers as Survivable Remote Site Telephony (SRST) gateways with MGCP fallback, and the use of Cisco Unified Communications Manager Express in SRST mode.


This section reviews the mechanisms for providing call survivability and device failover in remote sites. SRST and MGCP gateway fallback are the key components to deliver fail-safe communication services.

To use SRST as your fallback mode on an MGCP gateway, SRST and MGCP fallback must be configured on the same gateway. SIP SRST provides a basic set of features to SIP-based IP Phones. It has to be enabled and configured separately on Cisco IOS routers. In Cisco Unified SRST versions prior to 3.4, it provides a SIP Redirect Server function; in subsequent versions, it acts as a back-to-back user agent (B2BUA).

SRST uses SIP and H.323 dial-peer to provide backup when the CUCM is unavailable, regardless of the protocol used for communication to CUCM during normal operation. H.323 is the most prevalent default protocol used for SRST. Note: Additional licensing may be required for routers with IOS version 15.x.

CUCM Express in SRST mode provides more features to a smaller maximum number of Skinny-based IP Phones by falling back to CUCM Express mode.

MGCP gateway fallback configured as an individual feature can be used by a PSTN gateway if H.323 or SIP is configured as the default application, meaning that one of these peer-to-peer protocols will be used when MGCP loses connectivity to the call agent. However, as mentioned previously, H.323 is the default protocol used by IOS-based gateways.

Cisco Unified SRST enables routers to provide basic call-handling support for Cisco Unified IP Phones when they lose connection to remote primary, secondary, and tertiary CUCMs, as when the WAN connection is down in a centralized environment.

Cisco Unified SIP SRST provides backup to an external SIP proxy server by providing basic registrar and redirect server services or B2BUA services.

CUCM Express in SRST mode enables routers to provide basic call-handling support for Cisco Unified IP Phones if they lose connection to remote primary, secondary, and tertiary CUCMs, as when the WAN connection is down.

Cisco Unified SRST Operation

CUCM supports Cisco Unified IP Phones at remote sites attached to Cisco multiservice routers across the WAN. The following details the operation:

  • The remote-site IP Phones register with CUCM.
  • Keepalive messages are exchanged between IP Phones and the central CUCM across the WAN.
  • The CUCM at the main site handles the call processing for the branch IP Phones.
  • When the WAN link fails, the IP Phones lose contact with the central CUCM but then register with the local Cisco Unified SRST gateway.
  • The Cisco Unified SRST gateway detects newly registered IP Phones and queries these IP Phones for their configuration, which then autoconfigures itself.
  • The Cisco Unified SRST gateway uses Simple Network-Enabled Auto Provision (SNAP) technology to autoconfigure the branch office router to provide call processing for Cisco Unified IP Phones registered with the router.
  • Cisco Unified IP Phones attempt to reestablish a connection with the CUCM at the main site periodically when they are registered with a Cisco Unified SRST gateway, based on the keepalives sent to CUCM.

It typically takes three times the keepalive period for a phone to discover that its connection to CUCM has failed. The default keepalive period is 30 seconds.

MGCP Fallback Operation

MGCP gateways register with the CUCM. MGCP gateways exchange keepalive messages with the central CUCM across the WAN. CUCM is the MGCP call agent. The MGCP gateway performs a switchover to its default technology when the keepalives between CUCM and the Cisco MGCP gateway are missing. The switchback or re-home mechanism is triggered by the reestablishment of the TCP connection between CUCM and the Cisco MGCP gateway.

Dial Plan Requirements for MGCP Fallback and SRST Scenarios

SRST failover means that the remote site is independent from the complex dial plan implemented in CUCM at the main site. The SRST router needs to have a minimal dial plan implemented. This is accomplished using voip and pots dial peers.

During fallback, users should dial main-site directory numbers as usual. Because these calls must be routed over the PSTN during fallback, the main-site extensions must be translated to E.164 PSTN numbers at the PSTN gateway.

SRST use the local PSTN breakout, while supporting call preservation, autoprovisioning, and failover. To guarantee PSTN connectivity, dial peers with destination patterns corresponding to the PSTN access code must be implemented in H.323 or SIP gateways.

CUCM considers the remote-site phones as unregistered and cannot route calls to the affected IP Phone directory numbers. Therefore, if main-site users dial internal extensions during the IP WAN outage, the calls will fail. The Call Forward Unregistered (CFUR) configuration addresses this issue. CFUR provides additional call forwarding capability when the phone becomes unregistered for any reason.

Cisco Unified SRST 8.0 is supported in IOS gateways and CUCM Express version 15.0 (1). SRST 8.0 supports eight calls per line, E.164 numbering (using the + prefix), and five additional MOH stream when using SCCP. The number of phones supported in SRST 8.0 depends on the platform, with a maximum of 1500 phones supported on the 3945 platform.

To allow remote IP Phones to be reached from main-site IP Phones, CFUR must be configured for the remote-site phones. If a globalized dial plan is configured, CFUR can provides access to calling via a single Route Pattern (\+!) and CSS for all SRST calls. These calls are placed using the desired gateway via the PSTN; however, you an also use CFUR to direct the caller to a local extension, operator, or voice mail.

When using globalized call routing with Local Route Groups, the CFUR CSS is the same for all phones, so the local gateway is used for these calls. In the case of remote locations, you do not need to use the IP WAN to the main site and breakout to the PSTN for CFUR calls to other remote sites. The CFUR call uses the local gateway, resulting in an improved call routing.

Under normal conditions in multisite deployments with centralized call processing, calling privileges are implemented using partitions and calling search spaces (CSS) within CUCM. When IP WAN connectivity is lost between a branch site and the central site, Cisco Unified SRST takes control of the branch IP Phones, and the entire configuration related to partitions and CSSs is unavailable. Therefore, when running in SRST mode, the various classes of service within the branch router must be defined in the SRST router using the class of restriction (COR) functionality.

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