We have currently at work two ISP boxes(routers), i want to setup a firewall to protect an intranet on a local host. The problem is that not every one is using the same router.
Here is my question: how can i do to make all traffic be sent to one location (firewall-loadbalancer) then the loadbalancer switchs automatically between the two routers .
EDIT: The network is a small office network, there's 6 hosts and the intranet runs an one of them. The host must be reachable from outside but a firewall rule must filters the incoming connections. The rest of the network must send traffic to one locations, which forwards it automatically to the two routers.
EDIT2: Here is the current diagram : http://goo.gl/RBRvS
Solution: I found that i have either to use a bond interface on the firewall as pberlijn sayed, or routing manually to each router like here linux advanced routing