Concepts
Since I’m about to describe how to roll your own router, it will help if you understand how most TCP/IP routing works. Any router, whether Cisco, Proteon, or Linux, is based upon the premise that packets need to be forwarded. Why? Because, presumably, the packets that come in on one interface are not local to the other interfaces in the router. Therefore, the router in question must be able to take a packet, look at its destination, and forward it to the appropriate interface.
How does the router map network destinations to interfaces? Well, just as every router needs to be able to forward packets, so too it needs the ability to consult, build, and update a lookup table, called a routing table, that maps destination networks to interfaces.
If a packet comes in for network X, the routing table is consulted and the packet is dumped to the appropriate interface -- a local node that is either another gateway or the packet’s final destination. If network X does not exist in the routing table, the packet is dumped to the default destination, also denoted by IP address 0.0.0.0. If you have not specified a default destination, the packet is dropped, and an ICMP "Destination Unreachable" message is sent back to the originating network client.
How does the router maintain the routing table? In two ways: statically and dynamically. Static routes are added by you, in a known, and sometimes tedious, fashion. Typically, default routes are static routes.
Dynamic routes are "learned" routes, learned by some sort of routing protocol. For our installation, we were only interested in one routing protocol, RIP (Routing Information Protocol). It is simple, and widely available.
When a RIP router sees a RIP packet (which is a broadcast packet -- good to know for troubleshooting purposes), it updates the routing table so that future packets will follow the learned route to the new destination.
By the same token, the router "advertises" when it has a destination that other machines should know about. Obviously, routers that need to talk to each other need to speak the same protocol, or all is confusion!
Although RIP is okay for small networks with non-variable subnet masks, you will want to check out OSPF and BGP if you are about to embark on a complex networking plan. First of all, RIP is a broadcast protocol, which means your network is subject to broadcast storms. Second, RIPgives no "weight" to any particular route -- one route is considered as good as another. For example, RIP would consider your 9.6KBps backup link to a site to be just as good as a your primary 56KBps link.