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ISDN dial to two locations - questions about dialer interface behaviour

Hi, I have a couple of ISDN queries which I'd appreciate some assistance with - thanks to anybody who reads this and may be able to provide some direction.

One of my support contracts have lots of sites with routers that only use ISDN for WAN connectivity. They will use upto two bonded channels (depending on required BW) to dial into upto two hub locations (one being primary location, other for backup only if first is unavailable e.g. engaged/router dead/CHAP auth failure, etc.). I cannot use a single dialer interface configured with mutliple dial-up numbers as i'm using encryption and the crypto peer address will be different for each dial-in point.

If I work on the principle of using two dialer interfaces, with default routing out of each dialer interface, my questions are as follows:

1) If I default route out of each dialer interface, how can I predict without using a floating default route which dialer will be used first? Or is a floating default route the only way of blocking the second dialer from being used? e.g dialer 1 AD 20, dialer 2 AD 30.

2) If the "primary hub site" dialer fails e.g. the PRI has all channels used, will the router immediately move onto the second dialer which is admin distance costed higher? i.e. is dialer 1 interface declared similar to "line protocol down"

3) If the router dials into the primary hub site with the first channel, and the second channel tries to come up but fails, will it attempt to raise the second channel over the second dialer interface? I don't want a scenario where different channels are to both hub sites as this will create a multiple-path problem for inbound traffic which the existing routing protocol implementation cannot handle. Also, how can the primary dialer be declared "down" because a second channel cannot be raised when the initial channel would still be up?

My main question is should I be using admin distances to cost out the backup dialer route, and under what criteria will the router move to use the second dialer.

I have also thought about using the backup command to backup dialer1, with dialer2 - I trust in doing this I will only need a single default route out of dialer1?

Thanks in anticipation, regards, Neil Moss, BT Global Services

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