Moved from WAN Forum: Dual PRI T1s on seperate router/gateways

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Apr 17th, 2007
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I have an existing 3745 with 1 PRI connection (23b+d) into a NM-HDV2-T1/E1 card. We are in the process of purchasing a 2821 router with a VWIC2-1MFT-T1/E1. I am not planning on moving all the DSPs to one machine. The 2821 will also have a dual port fxo card for failover 911 calls.

I understand the basics of getting a single PRI line up and running and configuring the CCM to use it as the gateway. What I am looking for is a little direction on what else needs to be taken into account to ensure that both lines will be used. I understand that if they were on the same router then load balancing could happen and maybe a few other things. Is it still possible when using seperate routers? What let's the two routers inform each other or the CCM of failure or no lines left. The new line is supposed to be configured from AT&T as a 24d with 1 line able to act as the backup d when failover is needed. Are the controls for that to happen on AT&Ts side or is it something I need to configure.

MGCP is being used. I understand that CCM can utilize multiple gateways but I am looking for some insight on how call failover would work also we will be using 1 D channel for both PRIs if possible.

As always thank you in advance and I will rate helpful posts.

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Overall Rating: 4 (3 ratings)
mparekh Tue, 04/17/2007 - 14:21
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In order to use a single D Channel for both PRIs you will have to configure NFAS. NFAS is not supported using MGCP so you will have to configure the gateway as h323. Also both PRIs have to be on the same device to use the single D-channel.

See link for more info on NFAS:

Hope that helps.

paolo bevilacqua Tue, 04/17/2007 - 14:23
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I realize now that you have the second PRI provisioned with NFAS (D channel backup). In this case, first MGCP does not supports NFAS ASAIK, second, NFAS is supported with H.323 (and SIP I would believe), that requires that both links are terminated in the same router.

So in a way it would be easier if the second PRI could be provisioned with independent D-channel, that would give more freedom of using different protocols, gw, etc.

Hope this helps, if so please rate post

MattNicholls_2 Tue, 04/17/2007 - 14:32
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Thanks to both replies. I plan on keeping them seperate so the line I order will have to be a 23 + 1.

paolo bevilacqua Tue, 04/17/2007 - 14:36
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Thank you for the nice rating and good luck with your deployment.

lagos Tue, 04/17/2007 - 14:34
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Hi Matt,

I understand your question as how to get 2 PSTN gateways(PRIs in 2 separate routers) configured in callmanager in a redundant fashion.

The features that provide this capability are Route Groups/Route Lists. Put each gateway into it's own route group. Then create a new route list that contains both route groups.

Configure the route patterns to use this route list.

I would recommend to avoid NFAS all together. Any advantages of the single extra call in capacity are heavily outweighed by the unnecessary complexity of combing the D channels.

My .02,



Cisco Systems, Inc.


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