Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
New Member

ISDN Backup Issues

Hi all, I have some issues for ISDN, due to resource contraint, we want to subscribe as minimal of ISDN line as possible at our headquarter.

Here's the scenario. For example, if I have only 1 BRI port in my HQ, but it need to be catered for 2 remote sites. The 2 remote sites each running on a 128k leased-line. When one of the site LL failed, it is configured in such a way it will only establish 1st B-channel, when the threshold configured reach, then it will establish the 2nd B-channel. The problem starts here, if remote site A down and has been triggered the threshold, the BRI port at my HQ already consumed fully. What if now the remote site B LL failed?

I been thinking of Dial-backup is possible to solve my problem by watch both remote site A & B serial interface status. But that will require our HQ ISDN router to call out and we're going to bear the calling charge. If I strictly want the remote site is the one that initiate the call, can my objective be reached?

When remote site A down and fully consumed up the BRI at my HQ. Is there any configuration I can set in order to drop one channel reserved for remote site B if the remote site B LL failed at the same time?

Hope I get my question clear...appreciate if anyone out there can shed some lights to me....cheers


Re: ISDN Backup Issues


From the scenario mentioned in your post i tond think you can do the kinda reservation of one of the B-Channels to get the connectivity to other location.

You can tweak the dialer load threshold settings to get the second channel connected so that it can act as a reserve untill unless you see/get more traffic on the backup link.

AFAIK in general conditions when you for 128K at the HO and 128K in remote sites when both the primary links connected to the remote goes down you may face isdn disconnects when both the locations tries to connect or one of the remote site utilising the whole bandwidth.

i would assume that the possibilities of both the locations going down may be very less untill unless a major fault with the telco or majort shut in the telco premises.

Depending on the fault occurance u can plan off accrodingly and configure 2 dialers and give 128K bandwidth to the locations.

if the downtime is more and has an adverse effect in the business then its better to for one more isdn line and get seperate dedicated backup configured for the locations..


New Member

Re: ISDN Backup Issues

yup, I understand the chance both remote sites down at the same times is very less. Due to management issue, I need to think of the best way to maximize the bandwidth we have at the moment. (I dont think they plan to apply for the second ISDN to cater for one-to-one backup for the remote site though).

So can I say that it is impossible for the second remote site to establish the connection while one of the remote site consume the both available B-channel at the HQ. I can't do anything to tweak to configuration so that it'll detect the second call coming in, hence it'll drop one of the B-Channel in order to let the second remote site to establish the connection, at the very least both site get 64k and the connection is restored after the LL failed, performance is another issue though, most important is the connection is restored.

I guess that sum up the problem I'm facing, I do want a dedicated backup but unfortunately upper level can't understand my need.

New Member

Re: ISDN Backup Issues

anyone here can help? Thanks!

New Member

Re: ISDN Backup Issues

I interested to c the result as well

New Member

Re: ISDN Backup Issues

Its been a while, I've still yet to think of a new solution. If I have 8 BRI ports at one of our HQ router, it means I'll have 8 discrete phone number, can I bundled up 8 channels togethers and have one virtual number for the remote site to call upon?

I've heard virtual template, but don't know whether it can achieve what I want, for example with 8 BRI port, the phone number is:

i. A, B, C, D, E, F, G, and H

In remote site, supposely they need to call either one of the number, but can I bundled all the 8 ports (numbers) together, create a brand new number Z. Then all the remote site dialer will have the dialer string of Z, upon leased-line failure, the dialer string call to Z and use any B-channel available within the pool of 8 BRI ports...

is that achievable?

CreatePlease to create content