You may recall from previous chapters that in order for systems in one broadcast domain to communicate with systems in another broadcast domain, a router must be involved. A router usually acts as the demarcation point between broadcast domains. Remember that a broadcast domain is a Layer 2 concept. Routing, on the other hand, happens at Layer 3 of the OSI model – the Network Layer. We’ll look at how these two layers interact in a routed environment shortly.
The act of routing is primarily concerned with moving traffic between networks. This can be as simple as having two networks directly connected via a single router. More commonly, we will need to be concerned with moving traffic between networks across a larger internetwork – in other words, there may be many routers between a source and destination network. In order to get traffic from one network to another, we will need to somehow make routers aware of where they should next forward traffic, in order for it to reach its final destination. This can be done in different ways, each with associated pros and cons. Ultimately, the way in which we choose to configure our routers will depend on our goals in a specific network environment.
At the most basic level, a router does no more than make decisions about getting to a destination network. In that way, routing isn’t much different than driving a car between your home and work – there may be many ways to get to work, and ultimately you have to choose a path. At any given intersection, you have to make a decision. Should you turn the car and take a different path, or should you carry on and take what you usually consider to be the best route? Many factors will likely impact your decision. For example, you might consider the most scenic route, the one with the highest speed limit, or simply may be trying to avoid roadwork. Regardless of the route you choose, you ultimately make a decision at each major intersection on your drive. In the same way, routers are like intersections on a network – at each point, a decision has to be made as to where to send data next. There may be many ways to get to a destination, but ultimately the goal of routing is to determine the best path. What constitutes the “best” path depends on how your network is configured, as you will see shortly.