CUCM Call Routing (part 7): Route Lists

Continuing our call routing discussion, let’s move on to Route Lists now that we have discussed Route Patterns, Route Groups, Gateways, and Trunks. Recall that the logical processing order of these configuration elements are as follows: Route Pattern -> Route List -> Route Group -> Gateway/Trunk The configuration order of the call routing configuration must be bottom up. If we build a Route Group, but have no gateways or trunks to put in the Route Group, the Route Group has no value. If we build a Route List, but we do not have any Route Groups to point the Route List to… If we build a Route Pattern, but have no Route List to point the pattern to... but first let’s cover some fundamentals which will help paint a vision for what we’re looking to accomplish with the call routing elements. In a distributed call processing environment where there is more than one Cisco Unified Communications Manger (CUCM) cluster, there are normally multiple paths in which calls can be routed between sites. The first choice for call routing is normally to pass the voice call over the IP WAN, thereby reducing any PSTN toll charges. This type of call routing is commonly referred to as Toll Bypass because we are bypassing the toll charges required for placing traffic over the PSTN. This model is used to save money on international and long distance charges. If the IP WAN is unavailable for call routing, the call should be re-routed over the PSTN. Most deployments use an abbreviated dial plan for inter-site call routing. The number of digits that the dial plan can be abbreviated to is usually dependent on the size of the company and the direct inward dialing (DID) range used at each site. We will investigate an environment with two sites, each with a CUCM cluster. Each site will have a five digit internal dial plan and a seven digit inter-site dial plan. Site 1 will be located in New York City (NYC) and Site 2 will be located in San Jose, California (SJC). The DID range of phone numbers purchased for NYC is (212) 551-XXXX and the DID range for SJC is (408) 622-XXXX. In designing the inter-site dial plan, it was determined that the company could use a five digit inter-site dial plan, but they have planned the dial plan with the expectation of growth in the company. The SJC site will have a route pattern of 55.1XXXX pointed to the NYC Route List named NYC_RL. The NYC site will have a route pattern of 62.2XXXX pointed to the SJC Route List named SJC_RL. A Route List is a logical processing order of one or more Route Groups. The Route Lists mentioned above will each have two Route Groups. The first Route Group will route calls to an H.323 Gatekeeper over the IP WAN. The Gatekeeper will perform the important task of e.164 to IPv4 address translation and Call Admission Control (CAC). The CUCM server has no insight into the bandwidth available over the IP WAN. The network team in this deployment sized the Priority Queue (PQ) QoS mechanism to accommodate 50 G.729 calls. Without a CAC mechanism, CUCM could potentially route thousands of phone calls over this WAN link because CUCM believes that bandwidth is infinite. The H.323 gatekeeper will reject any calls over 50 phone calls so CUCM can re-route the call to the second Route Group pointed to gateways that will deliver the phone calls to the PSTN network. In the next blog, we will explore the digit manipulation techniques employed at the Route List level to ensure only 5 digits are passed over the IP WAN links, while a full 11-digit long distance number is delivered to the PSTN over our gateway interfaces.

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