Is it possible to use two VPN or MLPPP routers to aggregate, load balance, or fail over between WAN connections from a number of different providers?
Two conditions: 1. We need to keep the external IP constant so we can SSH/VPN in and out, run Exchange, etc. 2. No BGP. That's been the answer to similar questions, but a /24 IP block is not sensible for an office of twenty people.
I'm in an area where T1s and the like are insanely expensive, but WiMax, WiFi and DSL are available. We also get plenty of thunderstorms, so we can't rely on WiFi or WiMax alone. I currently use MLPPP over multiple DSL lines from the same provider, but that doesn't give us enough bandwidth.
You can only do transmit load balancing over the links. Without some form of coordination (ie BGP) others have no way is seeing the multiple return paths.
Theoretically, you could tunnel all your traffic over several VPNs, to a hosted server, and NAT everything behind that. Without the like of BGP, that's pretty much the only way to achieve what you're looking for though.
There are load balancer that can aggregate VPN, they will put vpn ipsec traffic from on site to another on all the available links, if one of the links goes down the load balancer will maintain the vpn up. there are several brands, i am familiar with the fiberlogic meshvpn unit