CUCM Path Selection

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 of4000. 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.

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

Figure 11-11 CUCM Call-Processing Logic

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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.

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.

NOTE A route group can be configured for circular distribution (round-robin) or top-down 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.

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 top-down 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

Figure 11-12 Route Groups

Cucm Circular Hunt Group

Devices:

Devices:

Cucm Circular Hunt Group

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

Add gateways and trunks to route group. Select distribution algorithm.

Figure 11-13 Route Group Configuration

Add gateways and trunks to route group. Select distribution algorithm.

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Order route group members (relevant if distribution algorithm top down is selected).

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 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 11-14 Route Lists

User Dials 9 1 408 526-4000

User Dials 9 1 408 526-4000

Output Peripheral Devices

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.

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.

Figure 11-15 Route List Configuration

Add route groups to route list.

Figure 11-15 Route List Configuration

Add route groups to route list.

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Order route list members (first entry has highest priority).

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

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 pre-dot 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.

More information about digit-manipulation configuration is provided in the next chapter.

Figure 11-16 Intercluster Call Routing Example

San Jose 408 526-4xxx 5-Digit Internal Dialing
Cisco Cucm Dialplan Pattern

Primary Voice Path: Intercluster Trunk Strip "52" and Deliver 64000 to Remote Cisco Unified CM

Philadelphia 215 555-1xxx 5-Digit Internal Dialing

Primary Voice Path: Intercluster Trunk Strip "52" and Deliver 64000 to Remote Cisco Unified CM

Philadelphia 215 555-1xxx 5-Digit Internal Dialing

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Responses

  • FRANCESCA
    When to use a Route pattern in cucm?
    1 month ago

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