GW design question

Unanswered Question
May 14th, 2007

PBX A has two PRIs and each connects to one of the two 2851 router's(R1 and R2) PRI port respectively. There are many sites like this connected via IP wan. GK is used to manage the E164 resolution and bandwidth control. Now since PBX A connects to two 2851, what's the best way to make sure two routers PRI can be used when call coming in from WAN? I have two solutions here:

1. GK always points to R1, when R1's 23 channel are full, R1 use another dial-peer to R2. But this requires tandem IP(h323- to -h323). I haven't used it before, not sure it will create some other issues. When both PRI full, the originator GW uses it's lower preferenced dialpeer to use local PBX

2. GK point to R1 and R2 with same priority using round robin fashion. So that two PRIs will be used equaly. When both PRI full, the originator GW uses it's lower preferenced dialpeer to use local PBX

I have this problem too.
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Paolo Bevilacqua Mon, 05/14/2007 - 10:15


Solution 2 is preferable. Simpler, load-balancing, and redundant.

Not sure what else the 2851 in your design does but you could do this with 2801s as well.

If you use a GK you may reasonably want redundancy there too.

ciscoforum Mon, 05/14/2007 - 10:43

only concern I have now is with sotulion 2 it looks like the PBX may not be able to send calls to these two routers with round robin fashion. So the chanlenge is the one of the PRIs could be full quicker than 2nd router.If this happens, there could be more calls bounced back to originator GW and re-route to PSTN which cause more long distance and this is not what customer wanted meantime the PRi on 2nd GW probably still have idle channels. I have no work around for this. I wish the PBX can round robin too. But the PBX technician seems can not gurantee on this. Any comment on this?

By the way I do have gk cluster in the design.

Paolo Bevilacqua Mon, 05/14/2007 - 11:13


I wouldn't be very concerned. The ISR are quite reliable hardware and I still have to see one fail in service, if that ever happen, the only thing you want is that router is turned off so being the T1 down, the PBX will not send more calls on it for sure.

ciscoforum Mon, 05/14/2007 - 16:21

Thanks for your response. But maybe I didn't clarify clearly. My concern is PBX can not program round robin, so the PBX will always send to Router A's PRI and then Router B's PRI when 1st PRI is full(This refers to when this PBX make calls to reach the other site via IP). If this the case, when GK sends round robin call to these two GWs, the call will be bounced back to originator when the call reach the Router A since the router A's PRI is full but meantime Router B may still have plenty idle channels. Make sense?

Paolo Bevilacqua Mon, 05/14/2007 - 16:40


Yes I did understood the scenario.

Honestly I don't know much about GK controlled calls, but I think there is a mechanism in which the GK is smart enough to stop sending calls to Router A, when it knows that there already 23 active calls, does not matter if incoming or outgoing. Even if no such mechanism, the no circuit disconnect should make the GK re-hunt to the other GW.

Also, you can send all calls to B initially and then to A, so that you will have kind of unidirectional GW's, A in one direction and B in the other, but with the possibility of overflowing to the other if necessary.

That would be similar to the traditional arrangement of top-down and bottom-up channel allocation on a circuit to minimize glare.


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