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Trunking with CUCM

Warning: I'm extremely new to CUCM.

I am curious as to what all is involved in connecting two CUCM (7.1.3 and other) systems together. By this I mean I have a CUCM setup here and someone else has it as well. I want people to be able to make calls between the systems (via IP).

I've stumbled upon SIP trunking as well as Gatekeeper and non-gatekeeper trunking. What are the advantages and disadvantages of each or is there a better way to go about this? Eventually I would like to scale this and allow calls between multiple CUCM systems.

3 Replies 3


For CUCM to CUCM trunking, in most cases, you would use an InterCluster Trunk.  This uses Cisco protocols to handling the trunk connection between the two systems.  A SIP trunk is simultaneously more complex and too simple; It is a protocol designed to allow any device that adheres to the SIP standard to speak to any other devices that adheres to the SIP standard.  Problem is, there's a lot of opinion about what exactly the "SIP standard" is.  For Cisco-to-Cisco connections, I would use the Cisco propritary solution.

Now, if you are going to be connecting a number of CUCM's together, that raises other issues.  You run into the "handshake problem".  You need to build and configure a connection between every two CUCM clusters.  This leads to N*(N-1)/2 connections, which is quite managable when N is 3 or 4, but becomes a hassle when N=10, and impossible when N=100.  That is when the Gatekeeper comes into play.  The Gatekeeper allows you to setup each cluster once, in the Gatekeeper, and the Gatekeeper plays traffic cop; Each cluster sends any non-local, on-net traffic to the Gatekeeper and the Gatekeeper directs it to the correct cluster.  It handles address resolution and call admission control, as well.

But to get started, all you need is to setup an ICT is the IP address of both ends.

We actually have a V2IU gatekeeper for Polycom video units. Could I use that as the gatekeeper and just start with it instead of having to switch to it later? We WILL have more than 1 or 2 other clusters routing calls back and forth if this works out as planned.

Also, i'm guessing only one gateway is needed or is one going to be needed at each location that has a cluster?

Jaime Valencia
Hall of Fame Cisco Employee Hall of Fame Cisco Employee
Hall of Fame Cisco Employee

For just 2 systems use an intercluster trunk non-GK controlled.

You can review pros and cons here

Cisco Unified CM Trunks



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