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Path selection is an essential dial plan element. After matching an entry in the call routing
table, CUCM has to select how and where to route the call. Most CUCM calls are routed
across IP trunks or gateways. CUCM allows multiple paths to be configured for a route
pattern for resiliency purposes.
Figure 11-11 shows a scenario in which a user has dialed a long-distance PSTN number
starting with an access code of 9, 1 for the long-distance operator, 408 for the area code,
followed by 526 for the exchange (office code), and a four-digit subscriber number of 4000.
These dialed digits have matched the route pattern of 9.14085264XXX. The primary path
for the call routing is the H.225 trunk over the IP WAN. The H.225 trunk is pointed to the
H.323 gatekeeper, which will provide call admission control in distributed multicluster
call-processing environments. If not enough bandwidth is available to route the call, the
H.323 gatekeeper returns an admission rejection, and the call is rerouted across the PRI
gateway to the PSTN.
270 Chapter 11: Call Routing Components
Path-Selection Elements
Route patterns are strings of digits and wildcards configured in CUCM. Route patterns can
point directly to a trunk or gateway device, but the device would not be available for any
other route patterns, and there cannot be redundancy if the device is not available or is out
of resources. It is best practice to point a route pattern to a route list logical entity. Route
lists are a prioritized list of route groups that allow digit manipulation to be configured on
a per-route group. A route group points to one or more devices that are selected based on a
distribution algorithm (circular or top down). The route list and route group elements
provide the greatest level of flexibility for call routing and digit manipulation.
Figure 11-11 displays the call routing logic of route pattern, route list, route groups, and
devices. The processing order is top down from the route pattern down to the devices, but
the configuration order is bottom up.
Figure 11-11 CUCM Call-Processing Logic
Path-Selection Configuration
To implement path selection in CUCM, the call processing logic must be built from the
bottom up. When the route group is created, devices are added to the route group. If the
devices do not exist yet, there will be nothing to relate to the route group. The following
steps have to be performed in the given order:
Step 1 Add devices (gateways and trunks).
Step 2 Build route groups from available devices.
Step 3 Build route lists from available route groups.
Step 4 Build route patterns pointing to route lists.
1 408 526-4000
Routers/Gateways
Gatekeeper
User Dials
9 1 408 526-4000
1001
Route
Pattern
San Jose
V
V
V
IP PSTN
9.14085264xxx
1
2
GK
IP
IP WAN
CUCM Path Selection 271
Route Group
A route group is a list of devices (gateways and trunks). It is recommended to put such
devices into the same route group that have identical digit-manipulation requirements,
because digit manipulation can be configured only once per route group during route list
configuration.
Figure 11-12 displays the call routing logic of CUCM. Notice that there are two gateway
resources to the PSTN in the route group. The route group can be configured with the topdown
distribution algorithm to use the resources of gateway 1 first and then the resources
of gateway 2. This configuration would prove useful in a scenario where gateway 1 and 2
are pointed to different service providers with different negotiated rates. If the same rate is
paid to both providers, circular routing is a viable solution that allows load sharing calls
across both gateways.
Figure 11-12 Route Groups
NOTE A route group can be configured for circular distribution (round-robin) or topdown
distribution. Circular distribution is used for load sharing resources, whereas the
top-down distribution is used to prioritize gateway usage within a route group. Multiple
gateway resources can be in the same route group.
First Choice Second Choice
Route
Group
Route
Group
IP WAN PSTN
Route
Pattern
Route
List
First Choice Second Choice
Route Pattern:
• Matches Dialed Number for External Calls
• Performs Digit Manipulation (Optional)
• Points to a Route List for Routing
Route List:
• First Level of Path Selection
• Performs Digit Manipulation
• Points to Prioritized Route Group(s)
Route Group:
• Second Level of Path Selection
• Points to the Actual Device(s)
Devices:
• Gateways (H.323, MGCP)
• Trunks (SIP, H.323)
Configuration Order
V
GK V V
272 Chapter 11: Call Routing Components
Figure 11-13 displays a screen capture of a Route Group Configuration page. To access this
configuration page, navigate to Call Routing > Route/Hunt > Route Group from CUCM
Administration. The route group should be given a descriptive name. If all the resources in
the route group will be used to access the PSTN and there is only one PSTN route group in
the CUCM cluster, use a name of PSTN_RG. Best practice is to use a naming nomenclature
that includes the configuration item’s functionality. The PSTN_RG route group name ends
with _RG to signify that the configuration item is a route group. Choose the distribution
mechanism from the Distribution Algorithm drop-down menu. Select the gateway or trunk
resource that you want to add to the route group from the Available Devices section of the
page and click the Add to Route Group button.
Figure 11-13 Route Group Configuration
Route List
A route list is a list of prioritized route groups. When configuring a route list, digit manipulation
can be set up per route group within the route list. Figure 11-14 is an example of
call routing where the first route group is an IP WAN route group distributing calls over a
trunk between clusters. If five-digit dialing is used internally and between sites, no digit
manipulation is needed at the IP WAN route group level. If the call is rejected by the H.323
gatekeeper or the IP WAN is down, the call is routed over the PSTN. The PSTN will not
route calls with five-digit dialing. The PSTN route group will need to prefix the necessary
number of digits to properly route the call. Assuming that 11-digit dialing is necessary to
Add gateways and trunks to route group.
Select distribution algorithm.
Order route group members (relevant if
distribution algorithm top down is selected).
CUCM Path Selection 273
route the call over the PSTN and direct inward dialing is in effect at the destination, six digits
are needed to be prefixed to the dialed digits to properly route the call over the PSTN.
Figure 11-14 Route Lists
Figure 11-15 displays a route list configuration in which two route groups have been added
to a route list. The route groups are in a prioritized order with the top route group being
the highest priority and the bottom routed group the lowest priority. IP-WAN_RG has the
highest priority, and PSTN_RG has the lower priority. If calls cannot be set up using a trunk
of the IP_WAN_RG route group, the PSTN_RG route group is used to route the call.
CUCM attempts to route the call from all the devices of that route group according to the
route group distribution algorithm (circular or top down). Use good naming nomenclature
when configuring route lists that identify the functionality of the route list. If the route list
is being used to route calls between New York and San Jose, the name SanJose_RL works
well.
First
Choice
User Dials
9 1 408 526-4000
Second
Choice
Route Pattern
9.14085264XXX
Route Group
PSTN
Route List
Route Group
IP WAN
GW2
PSTN
Trunk IP
GW1
IP
274 Chapter 11: Call Routing Components
Figure 11-15 Route List Configuration
At the bottom of the Route List Configuration page, the Route List Details hyperlinks can
be configured per route group. This is where digit manipulation can be configured for each
route group that is a member of the route list.
Figure 11-16 illustrates two sites of an enterprise (San Jose and Philadelphia) in which each
phone has a 5-digit extension and a corresponding 11-digit long-distance PSTN DID
number. CUCM has replaced an existing system in which users dialed seven digits for
all intersite calling. The dial plan is capable of using five-digit dialing to call between the
locations, but the solution will use seven digits because the existing system used seven
digits or the company expects a certain amount of expansion.
At the CUCM in Philadelphia, a route pattern of 52.64XXX is configured for intersite calls
toward San Jose. The route pattern points to a route list with two route groups. The primary
route group includes a trunk, and the secondary route group includes a group of PSTN
Add route groups to route list.
Order route list members (first entry has highest priority).
CUCM Path Selection 275
gateways. Depending on the chosen path, the following digit-manipulation requirements
apply for a call placed from Philadelphia to 526-4000:
■ Calls routed over the intercluster trunk: The first two digits (52) of the called
number (526-4000) have to be stripped so that the receiving CUCM in San Jose finds
the five-digit number as a configured directory number on one of its IP phones. In
addition, the calling party number has to be changed from a five-digit extension to a
seven-digit intersite route pattern (prefix 55 to calling party number). The placement
of the dot (.) is critical in this scenario. The called party DDI will be configured as predot
to remove the 52 from the dialed number. The result will forward only the dialed
digits 64000 to San Jose for digit analysis.
■ Calls routed over the PSTN: The called number has to be extended to a full 11-digit
long-distance number by prefixing 1408 to the dialed 7-digit number. The result is an
11-digit long-distance number of 1 408 526-4000. The San Jose gateway has been
configured with an inbound call routing significant digits set to 5. CUCM will analyze
only the last five digits of the received pattern. Alternatively, most providers customize
the number of dialed digits forwarded from the PSTN to the company’s dial plan
requirements. The circuit may have been ordered so that only five digits are received
from the PSTN.
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