I am setting up a short-haul WAN IP connection between two buildings. I have a 100Mbps wireless infared laser system as my primary link, and a leased T1 as my backup link. I plan to use Linux boxes as routers on both ends. Both the T1 and laser connections will be back-to-back between the two Linux boxes.
We expect that the laser system will go down on a regular basis, for example during heavy rain storms. I want to be able to revert to the T1 with no interruption to client machines and with the absolute minimum possible number of lost packets. The T1 is always up.
I know that this should be doable with routed, gated, etc, but I don't really know how they work. I know the basics of IP routing, the piece I'm missing is how to set up routing for two links so that packets travel over whichever one is running.
Can someone tell me how this is all supposed to work?