This topic describes how Cisco Unified SRST can provide survivability to remote phones.
- Communication between Cisco Unified Communications Manager and IP phones is broken.
- IP phones register with the local gateway (either Cisco Unified SRST or Cisco Unified Communications Manager Express in SRST mode).
Fallback for IP phones is provided by the Cisco Unified SRST feature and improves the availability of remote IP phones.
A WAN link connects IP phones at a remote site to the Cisco Unified Communications Manager at a central site, which is the call-processing device. If the WAN link fails, Cisco Unified SRST enables the gateway to provide call-processing services for IP phones. IP phones register with the gateway, which is listed as a backup Cisco Unified Communications Manager server in the server group configuration of the IP phones. The Cisco Unified SRST obtains the configuration of the IP phones and can route calls between the IP phones or out to the PSTN.
During normal operation of remote phones, while the connectivity between IP phones and their primary server (Cisco Unified Communications Manager) is acceptable, the following happens:
- Remote IP phones are registered with Cisco Unified Communications Manager over the IP WAN.
- Cisco Unified Communications Manager manages call processing for IP phones.
Fallback for IP Phones: Fallback Mode
The figure illustrates the operation of Cisco Unified SRST when IP phones lose connectivity with their primary Cisco Unified Communications Manager.
When Cisco IP phones lose contact with Cisco Unified Communications Manager, they register with the local Cisco Unified SRST router to sustain the call-processing capability that is necessary to place and receive calls.
The Cisco Unified SRST gateway automatically detects a failure, queries IP phones for configuration, and automatically configures itself. The Cisco Unified SRST gateway uses SNAP technology to autoconfigure the branch office router to provide call processing for Cisco IP phones that are registered with the router.
Cisco Unified Communications Manager Express in SRST mode can be used instead of standard Cisco Unified SRST functionality. In this case, IP phones register with Cisco Unified Communications Manager Express when they lose the connection to their primary Cisco Unified Communications Manager server. Cisco Unified Communications Manager Express in SRST mode provides more features than standard Cisco Unified SRST.
Call Forward Unregistered
Using CFUR to reach remote-site IP phones over the PSTN during WAN failure: The remote site lost connectivity to the main site. Phones are registered to the remote gateway:
- Cisco Unified Communications Manager for the main site does not route calls to the affected IP phone directory numbers.
- CFUR allows routing to alternate numbers for affected (unregistered) IP phones.
IP phones that are located at remote locations can use an SRST gateway as a backup for Cisco Unified Communications Manager in case of IP WAN failure. The gateway can use its local dial plan to route calls that are destined for the IP phones in the main site over the PSTN. But how should intersite calls be routed from the main site to the remote site while the IP WAN is down?
The problem in this case is that Cisco Unified Communications Manager does not consider any other entries in its dial plan if a dialed number matches a configured but unregistered directory number. Therefore, if users at the main site dial internal extensions during the IP WAN outage, their calls will fail (or go to voicemail). To allow remote IP phones to be reached from the IP phones at the main site, you can configure CFUR for the remote site phones. CFUR should be configured with the PSTN numbers that are used at the remote site so that internal calls for remote IP phones are forwarded to the appropriate PSTN number.
Using CFUR to Reach Users of Unregistered Software IP Phones on Other Devices
This subtopic describes how CFUR can be used to route calls to the cell phones (or any device) of a user who has shut down a PC that has a softphone installed.
Using CFUR to reach users of unregistered IP phone software on other devices: When a user at the main site shuts down a laptop with a softphone, such as Cisco Jabber, the following happens:
- The Cisco Unified Communications Manager of the main site does not route calls to the affected IP phone directory number.
- CFUR allows routing to alternate numbers of the user (for example, a cell phone number).
If a mobile user has a laptop with a softphone (for instance, Cisco IP Communicator) and shuts down the laptop, CFUR can be used to forward calls that are placed to the softphone to the cell phone of the user. The user does not have to set up CFA manually before closing the softphone application. However, if the softphone is not registered, calls are forwarded to the cell phone of the user. This action is another application of the CFUR feature that improves availability in Cisco Unified Communications Manager deployments.