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VOIP Gateway - Generate 'Busy Back' when WAN down?

Good evening all,

I've got a little problem, which I think I may have a solution to, but would like some help in confirming that what I'm suggesting is possible and some guidance on how to implement it.


I'm using a 3825 as a voip gateway router. It has 3 x Channelized E1/PRI ports which connect directly to our Rockwell ACD (Automatic Call Distribution) phone system. Calls are delivered (Q931) from the ACD to the 3825 router via these E1's.

From the 3825 router the calls are treated as H323 VOIP calls and are sent to a remote site across a WAN Serial link. The WAN link is MPLS, although the transport mechanism is irrelevant to the problem, it could quite easily be a point-to-point link.

At the far end of the WAN there is another 3825 router which converts the H323 calls back to Q931 to deliver them to another ACD via Channelized E1 ports.

This configuration works well until there is a WAN link failure. When the WAN fails, the Channelized E1 ports are unaffected (obviously) and so the Rockwell ACD is unable to recognise that there is any problems with routing calls. As far as it is concerned, the E1's are up and able to accept calls. However, this results in us dropping calls as they are obviously unable to route across the WAN.

What I would like to do is monitor the Serial interface and if it goes down, then shut down the Channelized E1 ports, which will generate a 'Busy Back' signal to the ACD and thus prevent it from trying to route calls via this route.

My solution (well the only one I could think of) would be to use Embedded Event Manager Policies Using Tcl, whereby a script would monitor the line protocol status of the Serial link and if it appears to go down, then run a CLI script to shut the Channelized E1 interfaces.

Is this possible and would it work? Are there any alternatives that I may have overlooked?

If it is possible, could someone supply a sample config for this scenario (I will freely admit my scripting skills are not the best!).

The key to the solution is that a 'Busy Back' signal needs to be sent to the ACD when the WAN is down and the only way I could think of doing that was to shut down the E1's.

Any help or guidence is very much appreciated.

Dom Wilkinson


Re: VOIP Gateway - Generate 'Busy Back' when WAN down?

I might not give you answer, but my question about MPLS. Are they fully mesh?? If the one of the WAN link, they should able to route to another link dynamically, right??

Aslo, I dont know about how your voice call route.. but can you use loopback interface for your H.323 call?? It is because, if the MPLS is fully mesh network, you can point to loopback interface on both end, it should able to route, right??

Just my stuipd $0.05 cents here.


New Member

Re: VOIP Gateway - Generate 'Busy Back' when WAN down?


Thanks for your reply, but unfortunately the serial link (singular)to the MPLS network is just a tail (i.e. last mile single connection), hence my comment about the MPLS connectivity being irrelevant. Effectively, it could be considered to be a single point-to-point link.

It would appear that I was thinking far too deeply as someone from another forum has pointed out that I should be able to use the Local Voice Busy Out (LVBO) or Advanced Voice Busy Out (AVBO) command set to achieve what I want to do.

How I managed to overlook these commands, I don't know! However I've been reading up on them and will attempt to implement them tomorrow.

I'll post back my findings and let you know if it works.


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