How Static Routing Works

Static routing is the process of adding IP routing information manually into the routing table. Let’s start by adding a static route to Router A so that it knows the 192.168.3.x network is accessible through Router D. If we were using RRAS as our routers we would type this command at a command prompt:

route add 192.168.3.0 mask 255.255.255.0 192.168.111.1 metric 1

All routers are configured differently, this is just how a Windows RRAS is done but the principle is the same regardless. This command adds an entry into the routing table that states the 192.168.3.x network can be accessed through IP 192.168.111.1. When PC1 sends data to 192.168.3.x it is forwarded to Router A but this time Router A finds a match to the subnet which tells it to forward the packets to 192.168.111.1 (Router D). Router D of course then delivers it directly to the machine. You may be asking yourself why we can’t just add this route directly on PC1 instead allowing us to bypass Router A? This is an option, and we would eliminate a hop in doing so. However, it is bad practice; remember this subnet could have hundreds of computers attached to it, it would mean adding the route to all of them. This is more work, and in addition it further complicates the network. For example what if due to network topology changes you are required to change Router D’s IP address in the near future? The route now needs updating but rather than update it once on a router you need to do it on every machine again! It is best leaving routing to the routers. Adding the route at the router means you only have to do it once.

Router A now has routes to 3 subnets; the 2 that are directly attached and the one we added above. We add another route for the last network of 192.168.2.x. We could add an entry telling Router A that the 192.168.2.x network can be accessed through Router B but since the 0.0.0.0 rule (Router A’s own default gateway) also sends packets to Router B it would be pointless. Instead we will add this new static route to go in the opposite direction for redundancy like so:

route add 192.168.2.0 mask 255.255.255.0 192.168.111.1 metric 2

As this is a more specific match than 0.0.0.0 ip routing will favour this first. When sending traffic to 192.168.2.x it will ALWAYS send it to Router D. If for any reason this fails it will then fall back to the 0.0.0.0 rule and send packets to 192.168.111.254.

Adding the two static routes above allows our router to “see” all 4 subnets. You would then configure the other 3 routers in the same manner.