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Interoffice Trunking with Elastix & IAX

The Scenario

This is scenario and configurations are taken from a production system, so you know that they work. One of the Best VOIP Solution.

The company has two offices one in Sydney and one in Melbourne. The office in Sydney has 60 staff and the one Melbourne has 20 staff

The two offices have a Virtual Private Network (VPN) between them which will generally be the case with most businesses that need to share or access data between their offices.

Elastix IP Address & IP Ranges used by each office
The Sydney office has a 192.168.0.0/255.255.255.0 ip address range
The Sydney Elastix system has an ip address of 192.168.0.100
The Melbourne office as a 192.168.1.0/255.255.255.0 ip address range
The Melbourne Elastix system has an ip address of 192.168.1.100

Extensions setup on Elastix at each Office

Sydney : Extensions 201 – 260
Melbourne : Extensions 501 – 521

IAX Trunk Setup

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On the Sydney Elastix system

Add an IAX Trunk

Leave all other options until you reach Trunk Name as default. I know it hurts to leave fields blank, resist the temptation until you have a working trunk, then come back and tweak these options. Enter all the details below listed under the Sydney Elastix System.

On the Melbourne Elastix System, Add an IAX Trunk, and enter the information listed under the Melbourne Elastix System

Sydney Elastix System Settings

Trunk Name : melbourne

PEER DETAILS

username=melbourne

type=peer

secret=yellow47

qualify=yes

host=192.168.1.100

context=from-internal

trunk=yes

disallow=all

allow=gsm

User Context : sydney

USER DETAILS

type=user

secret=yellow47

host=192.168.1.100

context=from-internal

disallow=all

allow=gsm

Registration

{Blank}

Melbourne Elastix System settings

Trunk name : sydney

PEER DETAILS

username=sydney

type=peer

secret=yellow47

qualify=yes

host=192.168.0.100

context=from-internal

trunk=yes

disallow=all

allow=gsm

User Context : melbourne

USER DETAILS

type=user

secret=yellow47

host=192.168.0.100

context=from-internal

disallow=all

allow=gsm

Registration

{Blank}

You should now be able to access the Asterisk CLI and type the command iax2 show peers on the Sydney Elastix system

So what have we done in the configurations that we have completed so far?

The Peer Details on the Sydney Elastix system, we have setup under Trunk Name Melbourne the details which include how to contact the Melbourne System, what password to use, the context to use, the codec to use, and we have also told it to trunk the calls. You can omit the Trunk=yes if you want, but it needs to be removed from both sides. Trunk=yes reduces the overhead a little especially if you have a number of calls being handled by that trunk.

The User Details on the Sydney System, are basically the authentication information for the Melbourne system when it tries to make contact with the Sydney system. The Sydney system will match the authentication being provided by the Melbourne system with the information here. If it matches then it will let the call come through.

The reverse is the same on the Melbourne Elastix System

It is as simple as this, you have setup a trunk between two offices. Now we need to get the Elastix systems on both ends to use this trunk because as it stands now, other than a but of authentication traffic, it doesn’t do much else. That’s where the next part of this tutorial comes into play.On both systems, we need to setup an outbound route to tell it what to do when a caller in Sydney wants to call an extension in Melbourne.

So, what we have completed here is an outbound route that tells the Sydney Elastix system, when a user calls any three digit extension that starts with a five(5) that it is to be directed to the IAX2/melbourne Trunk.

We have also ticked the Intra Company Route option which tells the system to preserve the CID information when transferring the call to the Melbourne system.

On the Melbourne System, setup basically a mirror image of the Sydney outbound route.

As you will note, the route name is called SydneyExts, again Intra Company Route is ticked, and the other main difference is that we are telling the Melbourne Elastix system to handle any Melbourne users that dial 2xx and redirect the call to the IAX2/sydney Trunk.

That is it. The best way to test it is to dial a Melbourne extension from the Sydney office. There is no other science needed, no magic potions. The main reason for failure for many is usually the failure of the IAX trunk. There are many configurations on websites, some that contain lines using the G729 codec only (which is common), but on freshly installed Elastix systems, you are generally not going to have the G729 codec loaded. My recommendation is leave the codec setting as GSM until you have completed basic testing.