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Does multiple sites using ISDN BRI backup require PRI at central router?

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Aug 9th, 2002
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I am investigating using ISDN BRI backup from spoke sites to the central hub router. If several sites need to use their ISDN backup circuits simultaneously, I am guessing that they need to dial into a central PRI to satisfy bandwidth requirements. Is this correct?

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tepatel Sat, 08/10/2002 - 10:50
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It depends how many spoke sites will dialin simultaneously.. You need to have a multiple BRI lines or a PRI line at central site depending on total b_channel requirement at central site when when all the spokes going to dialin simultaneously.

Now you can design a backup scheme using isdn like

a)isdn line will be used when the primary line is fully loaded to have more bandwidth

b)isdn line will only be used when primary line is down..

gbrome Mon, 08/12/2002 - 04:53
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I plan on only using the ISDN as a backup solution, versus using it for adittional capacity. I am planning for worst case scenario, where several sites would need 128K or better speed. I have seen a lot of documentation detailing BRI-BRI connections, but is there any source of details of BRI into PRI? For example, would each site dialing in need a different SPID number, or would they dial they same number and just use the channels they require depending on their bandwidth?

vcjones Mon, 08/12/2002 - 11:53
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You can use either a PRI or multiple BRIs, whichever is cheaper for the number of B-channels you need worst case. There are a number of configuration examples in my book High Availability Networking with Cisco, you can browse the listings on my website. Depending upon your telco, you may find the number of BRI's required to justify a PRI is surprisingly low, but the configuration is virtually identical unless you pick a configuration which does not permit you to expand to multiple PRIs, which is short sighted.

Good luck and have fun!

Vincent C Jones


gbrome Tue, 08/13/2002 - 04:47
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You've made a sale! Seriously, thanks for responding and pointing me towards the information I need. I am looking forward to receiving your book.

gbrome Mon, 08/19/2002 - 09:07
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Great book. Lots of thought provoking material. I did have a question on your use of the loopback interface for testing. Is pinging the loopback address on the remote router supposed to bring up the backup circuit? I have tried this, but no luck so far. Being able to do testing w/out bringing the primary Frame circuit down would be absolutely great.


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