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Trunk Groups - Why bother?

Jan. 08, 2010
Joe Parlas

One aspect of Unified Communications is this concept of trunk groups.  I will discuss what they are, what benefit they can provide and how to configure them for Cisco gateways.

First of all the official definition of a trunk group is “A group of trunks serving the same special purpose. The term commonly is applied to voice Private Branch Exchange (PBX) trunks. Multiple Direct Inward Dial (DID) trunks commonly are grouped together in a trunk group as well.”

For Cisco purposes, a trunk can be applied to Analog Circuits as well as Digital DS0's. The main benefit of using trunk groups is that the same search parameters as well as digit manipulation can be done for a Trunk group which could represent many FXO's or ISDN Channels for that matter. So instead of applying the same manipulation across many voice ports, I could simply use a trunk group; then for outbound calls, the same manipulations will be applied across all circuits. It does save quite a bit of configuration if you have multiple circuits that require the same treatment.

Also another benefit of using trunk groups is that it will reduce the number of outbound dial-peers I would have to create and I can directly control the hunting scheme as to which ISDN Channel or FXO port will be chosen first as well as the ability to dedicate some channels of ISDN circuit to particular dial-peers.

Now let's first look at the types of hunting schemes we can use with a Hunt Group.

  • Least-idle - Interface of a timeslot that has been idle the longest

  • Least-used - Interface with a timeslot that has the maximum unused time

  • Longest-idle - Interface with the timeslot that has been idle the longest

  • Random - Any interface with a timeslot within the Hunt group is chosen

  • Round-robin - The next interface in line with a free timeslot is chosen

  • Sequential - Interface of the highest preference is selected (more discussion on how to affect the highest preference)

Now as a reminder, hunt groups only affect how the gateway will process outbound calls so the dial-peers we keep referring to are outbound dial-peers which use destination-pattern for a match. You cannot control calls from the provider or PBX end coming into the gateway.

Now for the syntax of declaring a trunk group will be given in this first example which will be four FXO port connected to the Central Office for 911 calls.

First configure the trunk group in global config mode:

Router(config)#trunk group GKNEmergency

Router(config-t-trunk-group)#description 4 fxos for dial-out 911

Router(config-t-trunk-group)#max-calls voice 2 in

Router(config-t-trunk-group)#hunt-scheme random

Router(config-t-trunk-group)#translation-profile outgoing 911ANI

Now here I declared a trunk group called GKNEmergency and I am allowing inbound calls but only 2, my hunt scheme has been set to random and I have attached a outgoing translation profile to ensure I will be sending the correct ANI.

Now I will have to apply the hunt group to my FXO voice ports as per the following command: Hunt-Group {hunt group name} [optional preference number from 1 to 64 where 1 is the highest]

Router(config)# voice port 1/0/0

Router(config-voiceport)# trunk-group GKNEmergency 1

Router(config-voiceport)# exit

Router(config)# voice port 1/0/1

Router(config-voiceport)# trunk-group GKNEmergency 2

Router(config-voiceport)# exit

Router(config)# voice port 1/0/2

Router(config-voiceport)# trunk-group GKNEmergency 3

Router(config-voiceport)# exit

Router(config)# voice port 1/0/3

Router(config-voiceport)# trunk-group GKNEmergency 4

Router(config-voiceport)# exit

Now to add in the dial-peer to the equation:
Router(config)#dial-peer voice 911 pots

Router(config-dial-peer)#destination-pattern 911

Router(config-dial-peer)#prefix 911

Router(config-dial-peer)#trunkgroup GKNEmergency 1

Now the syntax for the POTS dial-peer is the same on the voice ports where you apply the trunk group name to reference to the outbound call and optionally a preference number in which ‘1’ would be the default.

On the next post we will address digital ISDN Channels using Trunk Groups.

Author: Joe Parlas