Survivable Remote Site Telephony
All Cisco endpoints register with CUCM by default unless there is another call control that can support the same or if an endpoint has specific call control requirements (for example, video endpoints that register with VCS). In the unlikely event of a WAN failure, the remote site endpoints need a backup call control. In such case, Cisco Unified survivable remote site telephony (SRST) can provide necessary call control features based on the Cisco IOS router, such as a Cisco Integrated Services Router (ISR) or Aggregation Services Router (ASR). Cisco Unified SRST (hereafter referred to as SRST) is an IOS gateway-based (license) feature that enables administrators to configure redundant call control for sites that do not have a local CUCM server. Cisco Unified SRST function can be co-resident with many other IOS features, including Cisco Unified Border Element (CUBE).
Cisco IOS gateways support two types of SRST:
- SRST (call manager fallback, traditional)
- Cisco Unified CME-based SRST or enhanced SRST (E-SRST)
The major difference between these is that traditional SRST offers basic telephony features and E-SRST provides an enhanced feature set for the endpoints. E-SRST builds on top of SRST by automatically synchronizing user, phone, hunt group, partitions, and so on from CUCM to Cisco Unified CME. Any change on CUCM is propagated to the branch sites without admin intervention. So when the WAN is down, the phone resembles the last changes performed on CUCM. When in E-SRST mode, Cisco Unified CME syncs automatically with CUCM to get updates as and when they happen on CUCM. Figure 5-3 depicts Cisco Unified SRST solution.
Figure 5-3 Cisco Unified SRST solution
The Figure 5-4 illustrates Cisco Unified CME-based SRST.
Figure 5-4 Cisco Unified CME SRST Solution
When the endpoints lose connectivity with CUCM (WAN failure or remote server is unavailable), SRST kicks in and allows the endpoints to register with the configured SRST reference—the IOS voice gateway (as defined in CUCM). In the case of traditional SRST, the router builds on the configured application service. The SRST gateway detects newly registered IP phones and queries these IP phones for their configuration, and then autoconfigures itself using the Simple Network Auto Provisioning (SNAP) technology. Only one instance of SRST is allowed per device pool in CUCM.
Cisco Unified SRST is mostly leveraged with Media Gateway Control Protocol (MGCP) fallback, as explained later in this chapter. Cisco Unified SRST supports the following features:
- Simple one-time configuration of basic SRST functions
- Customizable programmable line keys and button layout control
- Support for Forced Authorization Code (FAC)
- Normalized +E.164 support
In case the SRST reference is Cisco Unified CME at the remote site/branch office, the IOS router first searches for an existing configured ephone with the MAC address of the phone that tries to register. If an ephone is found, the stored configuration is used. The phone configuration settings provided by SNAP are not applied any ephone template. If no ephone is found for the MAC address of the registration phone, the router adds the ephone and applies the ephone template (similar to configuration using SNAP). Following are some E-SRST features that distinguish it from traditional SRST:
- Automatic provisioning of remote branch sites.
- E-SRST router is in sync with CUCM that pushes the updates to the branch routers. Automatic sync for moves, adds, and deletions from CUCM to router.
- GUI interface for provisioning, monitoring, reporting, and troubleshooting.
- On-demand information sync with CUCM.
Cisco Unified CME supports two modes in which it can be configured using E-SRST: Skinny Client Control Protocol (SCCP) and Session Initiation Protocol (SIP). This allows support for analog, SIP, and SCCP phones while in SRST mode.