tlug: linux multi BRI ISDN router - can it be built

["Andrew S. Howell" <andy@example.com>]

I am look at ways we can have multiple BRI ISDN lines for both
incoming and outgoing calls. I know there are many commercial
solutions for this, from Cisco, Accend, etc, but they all tend to be
rather expensive. TA's are getting to be dirt cheap here, so what I
was thinking was if there would be a way to have a linux box as a
router between our internal net and the TA's:

 internal net
 ------------------------------------
                 |
         +---------------+
         | Linux Router  |
         |               |
         +---------------+
                 |
 TA Net          |
 ------------------------------------
      |       |       |      |  
    +---+   +---+   +---+  +---+
    |TA |   |TA |   |TA |  |TA |
    +---+   +---+   +---+  +---+
      | a     |       |      |  
     BRI     BRI     BRI    BRI 

The problem is how to setup the routing? That is, lets say I want to
make an outgoing connection from the internal net. A TA has to be
configured to offer a route to that destination. But how do I do that?
I don't want to have them "pre configured", each with a separate set
of destinations. If I have TA a configured for clients A B and C, then
I out of luck when TA a is already talking to clients A and B, I
someone wants to connect to C.

Maybe if I used serial based TA's instead of 10BaseT ones. Still I
need some way for the linux box to advertise potential routes, and
then select some TA, set it up... Maybe I could use diald and some
scripts to track who is using what... 

Hmmm, the more I think about, hairier it gets. If only NTT would sell
a partial PRI, then I would any need a box to handle the one PRI,
which I think would be cheaper. I only need about 4 BRIs or so.


Thanks,

	Andy
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