This topic describes the remote site redundancy feature.
In a WAN failure, the Cisco Unified Communications Manager servers are unreachable for devices that are in remote sites. Service survivability is provided by two technologies:
- Cisco Unified or Cisco Unified Communications Manager Express in SRST mode provides call control services to remote IP phones.
- MGCP fallback provides voice gateway functions when the MGCP call agent is not in control of the media gateway.
Cisco Unified Communications Manager supports Cisco Unified IP phones at remote sites that are attached to Cisco multiservice routers across the WAN. Before Cisco Unified SRST was available, when the WAN connection between a router and the Cisco Unified Communications Manager failed or when connectivity with Cisco Unified Communications Manager was lost, Cisco Unified IP phones on the network became unusable during the failure.
Cisco Unified SRST overcomes this problem and ensures that Cisco Unified IP phones offer continuous (although minimal) service by providing call-processing support for Cisco Unified IP phones directly from the Cisco Unified SRST router. The system automatically detects a failure and uses 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 Cisco Unified Communications Manager subscriber is restored, call processing reverts to the primary Cisco Unified Communications Manager.
MGCP gateway fallback is a mechanism that allows a Cisco IOS router to continue to provide 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.
When to Use MGCP Fallback
MGCP fallback allows MGCP-controlled gateways to fall back to a default telephony application like H.323 or SIP:
- Might be used as a standalone feature to provide analog phones locally attached to a Cisco IOS router with survivability.
- Is most often used together with Cisco Unified SRST, which has been supported on the same Cisco IOS router since Cisco IOS Software Release 12.2(11)T.
- Provides gateway functionality to Cisco Unified IP phones in SRST mode.
- No call preservation for ISDN PRI or BRI interfaces.
MGCP gateway fallback that is configured as an individual feature can be used by a PSTN gateway if H.323 (or some other voice application) is configured as a backup service. Cisco Unified SRST and MGCP fallback must be configured on the same gateway if this single gateway should provide Cisco Unified SRST fallback service to phones and MGCP gateway fallback.
Although MGCP gateway fallback is most often used with Cisco Unified SRST to provide gateway functions to IP phones in Cisco Unified SRST mode, it can also be used as a standalone feature. An example is a fax application server that uses a PRI ISDN interface that is controlled by MGCP, where connectivity to the PSTN can be preserved by MGCP gateway fallback. Another example of an MGCP-fallback standalone configuration is a mechanism that allows analog interfaces that are controlled by SCCP to stay in service even when the WAN connection to the Cisco Unified Communications Manager is down.
MGCP gateway fallback preserves active calls from remote site IP phones to the PSTN when analog or CASprotocols are used. For ISDN protocols, call preservation is impossible, because Layer 3 of the ISDN stack is disconnected from the MGCP call agent and is restarted on the local Cisco IOS gateway. So, for active ISDN calls, all call-state information is lost when there is a switchover to fallback operation.
When to Use Cisco Unified SRST
Cisco Unified SRST provides limited call control service to SCCP and SIP phones in remote sites during a WAN outage:
- Supports secure voice fallback (if security is enabled).
- Allows simple, one-time configuration for SRST fallback service.
- Only basic telephony feature support:
- No message waiting indication
- No advanced features such as Cisco Extension Mobility or presence
Cisco Unified SRST provides Cisco Unified Communications Manager with fallback support for Cisco Unified IP phones that are attached to a Cisco router on a local network.
Cisco Unified SRST enables routers to provide basic call-processing support for Cisco Unified IP phones when they lose connections to remote primary, secondary, and tertiary Cisco Unified Communications Manager installations or when the WAN connection is down.
Cisco Unified SRST also supports security features. If IP phones are configured with security mode authenticated or encrypted in Cisco Unified Communications Manager and secure Cisco Unified SRST is deployed, the security features of Cisco Unified IP phones are preserved during fallback.
Cisco Unified SRST can support SIP phones with the standard RFC 3261 feature locally and across SIP WAN networks. With Cisco Unified SIP SRST, SIP phones can place calls across SIP networks in the same way that SCCP phones do.
Cisco Unified SIP SRST supports the following call combinations: SIP phone to SIP phone, SIP phone to PSTN or router voice port, SIP phone to SCCP phone, and SIP phone to WAN VoIP using SIP.
SIP proxy, registrar, and B2BUA servers are key components of a SIP VoIP network. These servers are usually located in the core of a VoIP network. If SIP phones that are located at remote sites at the edge of the VoIP network lose connectivity to the network core (because of a WAN outage), they may be unable to make or receive calls. Cisco Unified SIP SRST functionality on a SIP PSTN gateway provides service reliability for SIP-based IP phones in the event of a WAN outage. Cisco Unified SIP SRST enables the SIP IP phones to continue to make and receive calls to and from the PSTN and also to make and receive calls to and from other SIP IP phones.
When the IP WAN is up, the SIP phone registers with the SIP proxy server and establishes a connection to the B2BUA SIP registrar (B2BUA router). But any calls from the SIP phone go to the SIP proxy server through the WAN and out to the PSTN.
When the IP WAN has failed or the SIP proxy server has gone down, the call from the SIP phone cannot get to the SIP proxy server. The SIP phone instead goes through the B2BUA router and out to the PSTN.
The B2BUA acts as a user agent to both ends of a SIP call. The B2BUA is responsible for managing all SIP signaling between both ends of the call, from call establishment to termination. Each call is tracked from beginning to end, allowing the operators of the B2BUA to offer value-added features to the call. To SIP clients, the B2BUA acts as a user agent server on one side and as a user agent client on the other (back-to-back) side. The basic implementation of a B2BUA is defined in RFC 3261.
Cisco Unified SRST does not support enhanced features such as presence or Cisco Extension Mobility. MWI is also not supported in fallback mode.
When to Use Cisco Unified Communications Manager Express in SRST Mode
Cisco Unified Communications Manager Express in SRST mode provides backup telephony service during a WAN outage:
- Supports automatic Cisco Unified SRST configuration (basic features only).
- Additional features can be applied globally by templates or on a per-phone basis by manual configuration.
- All features available in Cisco Unified Communications Manager Express are supported.
- Features not supported by standard Cisco Unified SRST have to be statically configured (by templates or per phone).
- Mix of preconfigured phones and automatically configured phones is supported.
Cisco Unified Communications Manager Express in SRST mode enables routers to provide call-processing support to Cisco Unified IP phones by Cisco Unified Communications Manager Express if the phones lose connection to remote primary, secondary, and tertiary Cisco Unified Communications Manager installations or if the WAN connection is down.
When Cisco Unified SRST functionality is provided by Cisco Unified Communications Manager Express, you can use automatic provisioning of phones like with standard Cisco Unified SRST. However, due to the wide feature support of Cisco Unified Communications Manager Express, more features can be utilized, compared with standard Cisco Unified SRST.
Examples of features that are provided by Cisco Unified Communications Manager Express in SRST mode only are Call Park, presence, Cisco Extension Mobility, and access to Cisco Unity Voice Messaging services that use SCCP.
These features, however, cannot be configured automatically when a phone falls back to SRST mode. If a certain feature is applicable to all phones or directory numbers, the configuration can be applied by a corresponding template. If features have to be enabled on a per-phone (or per-directory number) basis, they have to be statically configured.
Phones that do not require unique feature configuration can be configured automatically so that only those phones that require individual configuration have to be statically configured in Cisco Unified Communications Manager Express.