How to avoid a Trombone/Hairpinning effect

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Aug 9th, 2010
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I have one 3825 device with 3 ISDN interfaces attached to one PSTN, one IVR and one PBX links.

PSTN ------------ Cisco Voice Gateway ------------------ IVR





There is how the calls flows and remains after the call are placed in:

Client>PSTN > VG > IVR > VG > PBX>Agents

How can i avoid a tromboning/hairping effect between the IVR and the Voice Gateway when the call are placed in?

There is how i want the calls remains after the are placed in:

Client>PSTN > VG >PBX>Agents

So,After all it can free up the channels between the Voice Gateway and the IVR.

Best Regards,


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iptuser55 Tue, 08/10/2010 - 03:17
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Normally the situation is on a PABX and it avoid tromboning by using PINX numbers and path replacement. Each PABX

should have a PINX number so the returning call is identified as coming back. Set in theory  set a  PINX number on CUCM so hopefully when the call is returned  the IVR system will send back the CUCM PINX number and so CUCM will recognise itself and drop the link betwen CUCM and IVR  ?????

elopesfilho Tue, 08/10/2010 - 06:07
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Thanks a lot for the answer! Unfortunately we don´t have an UCM running in this scenario. Do you know if it could it be done in the Voice Gateway Router?

Best Regards,


iptuser55 Tue, 08/10/2010 - 06:12
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What PBX are you running, most PABX should have the PINX setting particular as PINX comes from TDM

iptuser55 Tue, 08/10/2010 - 07:01
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You may need to use Look Ahead Routing or LAR- I think it is an Avaya "word". The Avaya engineer would be able to assist better

Steven Holl Tue, 08/10/2010 - 07:12
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If the circuit to the IVR is T1-CAS, and if the IVR can do a hookflash transfer, you can runt he h450-transfer script on the gateway to get it to process the transfer and pull the T1 call legs to the IVR out when the IVR makes the transfer.

If it is ISDN to the IVR, you can configure TBCT, but it requires writing a custom TCL script.  There isn't a TAC/cisco supported TBCT script for ISDN circuits currently, other than in a CVP environment.

elopesfilho Tue, 08/10/2010 - 07:42
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Hi Steven,

Thanks. It is an ISDN-EURO link between the IVR and VG.

Could this configuration solve the problem? Why do i need a TCL script to support it in this case?


1. enable

2. configure terminal

3. isdn switch-type switch-type

4. trunk group name

5. carrier-id name [cic]

6. isdn supp-service tbct [notify-on-clear]

7. exit

8. interface serial controller:timeslot

9. trunk-group name [preference_num]

10. exit

11. Repeat Steps 8 to 10 for each interface added to the trunk group.

Steven Holl Tue, 08/10/2010 - 07:46
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That command jsut enables a TCL script to be able to call the TBCT function.  The presence of that command alone will not enable TBCT on a PRI.

There is no native support in IOS for TBCT in the IOS call control API, which is why a TCL script is necessary.

Your options are to either switch to a CAS link or get a custom TCL script written for TBCT.  Cisco Developer Support may be able to help with the latter.



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