hi ng,

i need to backup a laser link connection bedween two buildings. this laser
link has got a 100Mbit interface one the one side. now my backup line runs
over a HDSL modem link with a 10Mbit interface, with a maximum throughput of
about 2-4Mbit over the modem line. now just in these days we got a lot of
fog! and no link via laser! :-(

ok, these laser devices always got the 100MBit interface up and running
while you do not see when they loose there link to the other side of the
laser. the connected device to that laser interface will never see that the
line is down since its direct neighbouring interface(laser) is always
showing as the link where up. the second, the backup line is far to slow but
more reliable.

the for me unknown device now has to check not the link status, but the link
throughput of the lines and decide on these circumstances how to split up
the traffic.
both lines should get some traffic but the laser, when there is throuhput
should take more load the the slow line. some kind of loadsharing.
i've got on both sides of these buildings more then around 1000 MAC adresses
with witch this device has to deal.

My idea is about building some linux box with some layer 2 forwarding
mechanics for both buildings one each. but now my question is where do i
start? if you got other suggestions how to solve this please bring youre
ideas! i just currently run out of ideas how to solve this one.

What i already tried is spanningtree configurations (spanncosts) via cat
3500xl in witch i get loops and some weird behavoir.
I also tried ciscos etherchannel (paga proto), but it does not work out with
diffrent link speeds. what you have to know about etherchannels is that you
can not even trick em with the link speeds! i mean by hanging a small switch
100Mbit switch in front of the 10Mbit Modem interface, simulating a 100Mbit
line!

so etherchannels need the same speed and same troughput on each line, witch
is bundled together as a big fat pipe. otherwise the bundle will not go up
or run as you wish. that is what i found out for me, if this is not true or
if someone knows diffrences please tell me or correct my given info.
following is a small overview of the situation:

network of building 1 - unknown device - Laserlink - unknown device -->
network building 2
network of building 1 - unknown device - Modem - unknown device --> network
of building 2

as example: one plug should go into the cisco of building one, the two other
links are connected to the laser and the modem. in the other building the
same physical buildup. i would be pleased to get some google keywords or
suggestions.

thank you greetings colin