CUCM Call Routing (Part 6): Trunks and Route Groups

The last blog introduced the four trunk types used in Cisco Unified Communications Manager (CUCM) to route calls over an IP network. The four types are as follows:

• Non-Gatekeeper Controlled Inter-Cluster Trunk

• Gatekeeper Controlled Inter-Cluster Trunk

• H.225 Trunk

• SIP Trunk

The H.225 trunk is a replacement for the Gatekeeper Controlled Inter-Cluster Trunk and has been available since CUCM 3.2. The Gatekeeper Controlled Inter-Cluster Trunk has not been removed from CUCM to ensure backward compatibility in upgrade situations. The H.225 Trunk is near identical to a Gatekeeper Controlled Inter-Cluster Trunk, but it will allow CUCM to route calls to an H.323 gatekeeper or and H.323 gateway. H.225 trunks are the preferred way of routing calls to and from a site using Cisco Unified Communications Manager Express (CUCME). The CUCME router will need a VoIP dial peer that logically points to CUCM.

The SIP Trunk allows CUCM to route calls to system running RFC3261 SIPv2. Session Initiation Protocol (SIP) allows the customer to include a Cisco Unified Presence (CUP) server which will allow a watcher to view the availability status of a presence entity (Cisco IP Phone). The industry standard SIP protocol uses Session Description Protocol (SDP) as a mechanism negotiate audio codec capabilities.

The beginning of the call routing processing logic in CUCM begins with the Route Pattern. A route pattern can point directly to a gateway or trunk. If there was one single ISDN PRI in a single site model and a very simple dial plan involving the 9.@ route pattern, the route pattern could be pointed directly to a gateway. If more than one PRI or gateway routers will be used for call routing, the gateways must be placed in one or more Route Groups. Route Groups are logical entities that allow multiple gateways to be placed in one route group. Within the route group, there are two distribution algorithms: Circular or Top Down.

An explanation of the Route Group distribution algorithms is best explained with an example. Global Knowledge (GK) decides to purchase three ISDN PRIs from three different PSTN service providers. Provider A charges 1 cent per minute for long distance calls within the continental United States. Provider B charged 2 cents per minute for the same types of calls, while provider C charges 3 cents per minute. GK wants to route all outgoing calls to provider A until all 23 B channels of the PRI are used. The twenty-fourth call should be routed to provider B. Any calls over the capacity of provider A and B (23 B channels X 2 PRIs = 46 phone calls), should be routed to provider C.

The Top Down distribution algorithm is the only logical choice to meet this objective. If all three providers were charging the same price and had the same five 9s of reliability, the circular distribution algorithm could be used to load share outgoing calls across all three PRIs. The incoming call routing is determined by the service providers.

The processing order of the call routing in CUCM is as follows:

Route Pattern -> Route List -> Route Group -> Gateway/Trunk

The configuration order of the call routing is bottom up. We have spoken about gateways/trunks and route groups. The next blog will discuss the Route List configuration options and inter cluster call routing.

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