Best use of multiple ADSL connections
We're looking at how best to increase the amount of bandwidth the school has access to, both for upload and download speed. We currently have one ADSL line at one end of the school that we use, and a second that I planned to get working with a nice new SmoothWall install.
We now maybe have a bit more money to spend on this, so I'm looking at getting a third ADSL line installed next to the second one and taking advantage of our ISP's offer of two uncontended ADSL 2+ lines running at 9Mb/s download speed each.
We want to make best use of all the bandwidth we'll have available. We'll have two lines next to each other offering 9Mb/s download and maybe 1Mb/s upload, and one line at the other end of the school offering 1Mb/s download and 512Kb/s(?) upload.
What's the best solution? Obviously, we want any web access to go through a filter - seemingly we can have multiple copies of SmoothWall installed throughout the school, so that's not a problem. At some point, a load-balancing router would probably come in handy. Routers that sit in the centre of a network seem to be rather expensive, around £3,000+, but routers that sit at the edge of a network, where the Internet connection comes in, seem to be quite cheap, and we can probably just set up a Linux VM to act as a load-balancing router.
Could I set up a load-balancing router with three network interfaces and have each seperate ADSL router port-forward incoming requests to a given network interface? If I do that, wouldn't I then have separate routers available on the network which users could connect to to get un-filtered Internet access? Can I set an ADSL router to refuse connections from anything except a trusted source? Can this only be done by (spoofable) MAC address? Is a VLAN the answer instead - set up a VLAN to take traffic from the ADSL router at one end of the school to the load-balancing router at the other end? Would this use up an excessive amount of our 2Gb/s backbone bandwidth?