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Cisco networking - using ISDN as backup internet connection

Our network currenlty connects to the internet via a 512 fractional T1 through a Cisco 1720 router. We also have 3 isdn lines in house that are used exclusively for video-conferencing. The ISDN lines connect to an ADTRAN NT1 (4 ports) which then connects to a Cisco IPVC video gateway. I want to know if there is a way (I'm sure some add'l hardware will be required) to use the ISDN lines (or a single line at least) as a backup internet connection when the T1 is down (unfortunately this has happened twice now in the past 3 months...). As always any and all help from the EE community is greatly appreciated.
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zuzzy

Well I'm not sure about splitting ISDN lines but I dont see why it would be a problem.  Certainly 2500 routers can have a backup command added in "conf t","int [port]" settings and we use it all the time.  the netowrk shoudl come up if the other end tries to route into the isdn as a backup but doesnt dial.  Look on the cisco cite for more info.

--Zuzzy
Sorry IE decided to ignore my stop command! I was going to add ont he end that a 1750 is similar to a 2500 as far a I know, so it should be the same.
It definitely is possible what you will need is an ISDN card in the 1720 and a dialup account to an ISP.
Do you want this to be a dynamic or manual switch over? Do you have mail coming into this router?
Some ISP will provide ISDN backup for you as an extra service.
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The biggest part of my concern, other than it in fact being possible to do this, is the physical connections between devices. Is it possible to use 3 ISDN lines to share the load as backup to the T1...? As i said earlier it's 3 ISDN lines that connect direct to a single NT1 - the ISDN lines from there run direct to the video gateway. I would want this to be a dynamic configuration... Email is coming in through this router as well. I'm not concerened w/ the ISP providing the backup since it's the ISP i've got the problems with... besides that the goal is to use the existing ISDN we have since it sits idle when a video conference is not in progress - rather than just add an additional wan link for redundancy alone. I wish there was a way to upload graphics with these questions so I could be sure the physical layout was clear.
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hstiles

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it's necessary to keep 3 lines to support the full possible bandwidth (384) for video... even if only 1 ISDN could be used for the redundancy that would be an improvement - the trick is keeping that one available for video as well at the same time.
An ISDN2 should have two RJ45 ports, so just plug a cable from each device into it
This is the setup of the current NT1 device: http://www.adtran.com/adtranpx/Doc/0/ITS4PI98BJAH39QA038BE81ID8/ITS4PI98BJAH39QA038BE81ID8.jpg

each of the 3 ISDN's run from the jacks to a U-interface (1 through 3 - #4 is empty). The 3 "S" interfaces are cabled directly to the IPVC gateway BRI interfaces.
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