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You can't split a particular IP connection
between two links, but can instead only determine which link a
particular connection will occur on. Given this, it sounds like you
want to have some way to detect that Link A is already saturated and then send
all further connections to Link B until Link A is no longer
saturated.
Maybe someone can tell you how to do that
if that's really what you want to do (others here know far more about this than
me), but my guess is you really don't want to do that. With the
huge bandwidth disparity between the two links, route cacheing, and
the inability know how much bandwidth any particular conneciton will
consume, I think you'd end up with a giant mess... those people with connections
unlucky enough to end up on Link A would probably be very unhappy people
indeed.
Generally speaking I think it would make
more sense to put all traffic over Link B, and then use Link A only for
emergencies. Maybe route the most critical traffic over Link A if you
really want to feel like its being utilized as something other than a pure
backup, but personally I wouldn't even do that.
Just because Link A is more reliable and
more expensive doesn't mean it makes sense to use it as your primary
conduit. With Link B having eight times the bandwidth, it
seems the obvious choice as the primary. Use it, and keep the users
happy most of the time (instead of making them miserable most of the
time). On the rare ocassions it goes down, use bandwidth shaping to make
sure the highest priority traffic gets access to Link A first.
In all the time I've used DSL, I've
had severe outages twice for reasons other than standard
maintenance. In both cases (in two separate locations), the cause was the
ILEC phone company mistakingly dropping the wire pair while doing other work
(freakin took over a week in each case to get my connectivity back!!).
This sort of thing could just as easily happen with a leased line though, so I'm
not really sure I buy that the leased line is really more reliable than DSL line
from a high quality ISP. Although maybe a particular SLA makes it so
in some legal sense since you can then sue someone. Personally, if your
leased line really costs more than the DSL, I'd get rid of it and get a 2nd DSL
line from another provider and use that as your backup instead.
Anyway, I guess my main point is that the
high cost of your leased line might be clouding your thinking on this. I
wouldn't let the comparitive cost be your guiding light here. Go with
what makes sense from a technology perspective, and don't guilt yourself into
trying to get full utilization out of the slow link just because it costs
more.
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