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Figure 5-11 illustrates requirements of standalone dial plans to work with MGCP fallback
and SRST.
SRST failover leaves the remote site independent from the complex dial plan implemented
in CUCM at the main site. The SRST router needs to have a dial plan implemented to allow
all remote-site phones, all main-site phones, and all PSTN destinations to be reached with
the same numbers as in standard mode.
During fallback, users should be able to dial main-site directory numbers as usual. Because
these calls have to be routed over the PSTN during fallback, main-site extensions have to
be translated to E. 164 PSTN numbers at the PSTN gateway.
114 Chapter 5: Examining Remote-Site Redundancy Options
Figure 5-11 SRST Dial Plan Requirements for Calls from the Remote Site
Most enterprises limit the range of destinations that can be reached from specific extensions
by applying class of service to the extensions. This limitation should still be valid during
times in SRST mode by applying IOS class of restriction (COR), as described in the next
chapter.
Ensuring Connectivity for Remote Sites
When SRST is active, you must take several measures to ensure connectivity from remote
sites to PSTN destinations, between different sites, and inside the site itself.
To guarantee PSTN connectivity, dial peers with destination patterns corresponding to the
PSTN access code have to be implemented. In H.323 or SIP gateways, these dial peers must
be present for normal operation. When MGCP gateways are used, dial peers are activated
by the MGCP-gateway-fallback mechanism. Interdigit timeout adopts open numbering
plans that do not have a fixed number of digits.
Voice translation profiles that are applied to dial peers, the voice interface, or the voice port
modify the calling party ID to enable callback from call lists.
For intrasite and intersite connectivity, voice translation profiles are configured to expand
called numbers to PSTN format during fallback.
The Cisco IOS command dialplan-pattern in call-manager-fallback configuration mode
modifies incoming called numbers to match the remote-site extensions. It ensures that
internal extensions can be dialed even though the lines are configured with the site code and
extension. The Line Text Label settings defined in CUCM are not applied to the SRST
phones, so the complete directory number applied to the line is visible to the user.
Dial Plan Requirements for MGCP Fallback and SRST Scenarios 115
Remote Site
Main Site 1001
CUCM considers the remote-site phones 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 or go to voice mail.
To allow remote IP Phones to be reached from main-site IP Phones, Call Forward Unregistered
(CFUR) can be configured for the remote-site phones. 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.
NOTE In older versions of CCM that did not support CFUR, it was not possible to
allow a main-site phone registered to CCM to call a remote-site phone in SRST mode
over the PSTN during a WAN failure. This was because CCM did not have a mechanism
to route calls to unregistered DNs through the PSTN.
CFUR Considerations
CFUR was first implemented in CCM Release 4.2.
Ensuring Connectivity from the Main Site Using
Call Forward Unregistered
During fallback, main-site users should still be able to call remote-site users using their
extension numbers, as shown in Figure 5-12.
Figure 5-12 Ensure Connectivity from the Main Site Using CFUR
116 Chapter 5: Examining Remote-Site Redundancy Options
As mentioned earlier, the CFUR feature allows calls placed to a temporarily unregistered
IP Phone to be rerouted to a configurable number. The configuration of CFUR has two main
elements:
• Destination selection: When the directory number is unregistered, calls can be rerouted
to voice mail or to the directory number that was used to reach the IP Phone through
the PSTN.
• Calling Search Space (CSS): CUCM attempts to route the call to the configured
destination number using the CFUR CSS of the directory number that was called. The
CFUR CSS is configured on the target IP Phone and is used by all devices that are
calling the unregistered IP Phone. This means that all calling devices use the same
combination of route pattern, route list, route group, and gateway to place the call. In
addition, all CFUR calls to a given unregistered device are routed through the same
unique gateway, regardless of the location of the calling IP Phone. It is recommended
that you select a centralized gateway as the egress point to the PSTN for CFUR calls
and configure the CFUR CSS to route calls to the CFUR destination through this
centralized gateway.
If an IP Phone is unregistered while the gateway that is associated with the direct
inward dialing (DID) number of that phone is still under the control of CUCM, CFUR
functionality can result in telephony routing loops. For example, if an IP Phone is
simply disconnected from the network, the initial call to the phone would prompt
the system to attempt a CFUR call to the DID of the phone through the PSTN. The
resulting incoming PSTN call would in turn trigger another CFUR attempt to reach the
directory number of the same phone, triggering yet another CFUR call from the central
PSTN gateway through the PSTN. This cycle potentially could repeat itself until system
resources are exhausted.
The CUCM service parameter Max Forward UnRegistered Hops To DN in the Clusterwide
Parameters (Feature—Forward) section in CUCM Administration controls the maximum
number of CFUR calls that are allowed for a directory number at one time. The default
value of 0 means that the counter is disabled. If any directory numbers are configured to
reroute CFUR calls through the PSTN, loop prevention is required. Configuring this service
parameter to a value of 1 would stop CFUR attempts as soon as a single call were placed
through the CFUR mechanism. This setting would also allow only one call to be forwarded
to voice mail, if CFUR is so configured. Configuring this service parameter to a value of 2
would allow up to two simultaneous callers to reach the voice mail of a directory number
whose CFUR setting is configured for voice mail. It would also limit potential loops to two
for directory numbers whose CFUR configuration sends calls through the PSTN.
Dial Plan Requirements for MGCP Fallback and SRST Scenarios 117
NOTE CUCM Extension Mobility directory numbers should not be configured to send
CFUR calls to the PSTN DID that is associated with the directory number. The directory
numbers of CUCM Extension Mobility profiles in the logged-out state are deemed to be
unregistered. Therefore, any calls to the PSTN DID number of a logged-out directory
number would trigger a routing loop. To ensure that calls made to CUCM Extension
Mobility directory numbers in the logged-out state are sent to voice mail, their
corresponding CFUR parameters must be configured to send calls to voice mail.
Keeping Calling Privileges Active in SRST Mode
Under normal conditions in multisite deployments with centralized call processing, calling
privileges are implemented using partitions and CSSs within CUCM.
However, 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 that
is related to partitions and CSSs is unavailable until IP WAN connectivity is restored.
Therefore, it is desirable to implement classes of service within the branch router when
running in SRST mode.
For this application, you must define classes of service in Cisco IOS routers using the class
of restriction (COR) functionality. You can adapt the COR functionality to replicate the
CUCM concepts of partitions and CSSs by following these main guidelines:
• Named tags have to be defined for each type of call that you want to distinguish.
• Basic outgoing COR lists containing a single tag each have to be assigned to the
outgoing dial peers that should not be available to all users. These outgoing COR lists
are equivalent to partitions in CUCM.
• Complex incoming COR lists containing one or more tags have to be assigned to the
directory numbers that belong to the various classes of service.
SRST Dial Plan Example
Call-routing components on Cisco IOS routers and CUCM are necessary before a dial plan
will work in SRST mode, as shown in Figure 5-13.
118 Chapter 5: Examining Remote-Site Redundancy Options
Figure 5-13 SRST Dial Plan Example
CFUR must be defined on the CUCM side. Configuring the Cisco IOS router is a little more
complex when you use dial peers, COR, dial plan pattern, and voice translation profiles to
define the simplified SRST dial plan. Note how this example lets you dial 9 to get out to all
numbers on the PSTN from the remote site but limits 900 calls with COR to align with the
same restrictions set in CUCM.
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