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As discussed, IP Phones located at remote locations can use an SRST gateway as a backup
for CUCM in case of IP WAN failure. The gateway can use its local dial plan to route calls
destined for the IP Phones in the main site over the PSTN. But how should intersite calls
be routed from the main to the remote site while the IP WAN is down?
The problem in this case is that CUCM 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 fail (or
go to voice mail). To allow remote IP Phones to be reached from the IP Phones at the main
site, configure CFUR for the remote-site phones, as shown in Figure 2-15. CFUR should
be configured with the PSTN number of the remote-site gateway so that internal calls for
remote IP Phones get forwarded to the appropriate PSTN number.
Figure 2-15 Using CFUR to Reach Remote-Site IP Phones Over the PSTN During WAN Failure
Main Site Remote Site
Using CFUR to Reach Users on Cell Phones
If a mobile user has a laptop with a softphone (for instance, Cisco IP Communicator) and
the user shuts down the laptop, CFUR can be used to forward calls placed to the softphone
to the user's cell phone, as illustrated in Figure 2-16. The user does not have to set up Call
Forward All (CPA) manually before closing the softphone application. However, if the
softphone is not registered, calls are forwarded to the user's cell phone. This is another
application of the CFUR feature that improves availability in CUCM deployments.
Availability 43
Figure 2-16 Using CFUR to Reach Users of Unregistered Software IP Phones on
Their Cell Phones
AARandCFNB
If a call over the IP WAN to another IP Phone in the same cluster is not admitted by CAC,
the call can be rerouted over the PSTN using AAR, as shown in Figure 2-17. The AAR feature
includes a CFNB option that allows the alternate number to be set for each IP Phone.
In the example, because the remote site does not have PSTN access, the call is not rerouted
to the IP Phone over the PSTN (instead of over the IP WAN). It is alternately rerouted to
the cell phone of the affected user. AAR and CFNB improve availability in multisite environments
by providing the ability to reroute on-net calls that failed CAC over the PSTN.
Figure 2-17 AAR and CFNB
Chapter 2: Identifying Multisite Deployment Solutions
Mobility Solutions
This section provides an overview of mobility solutions that solve issues that are the result
of roaming users and devices and multiple telephones (office phone, cell phone, home
phone, and so on).
When users or devices roam between sites, issues arise that can be solved by mobility
solutions:
• Device Mobility: Solves issues caused by roaming devices, including invalid device
configuration settings such as regions, locations, SRST reference, AAR groups,
Calling Search Spaces (CSS), and so on. The Device Mobility feature of CUCM allows
device settings that depend on the physical location of the device to be automatically
overwritten if the device appears in a different physical location.
• CUCM Extension Mobility: Solves issues that are the result of roaming users using
shared guest IP phones located in other offices. Issues include wrong directory number,
missing IP Phone service subscriptions, CSS, and so on. CUCM Extension Mobility
allows users to log in to guest phones and replace the IP phone's configuration with the
IP Phone configuration of the logged-in user.
• Cisco Unified Mobility: Solves issues of having multiple phones and consequently
multiple phone numbers, such as an office phone, cell phone, home (office) phone, and
so on. Cisco Unified Mobility allows users to be reached by a single number, regardless
of the phone that is actually used.
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