A network node, which is just a device that forwards packets toward a destination, can be a router, bridge, or switch. They operate on different layers of a networking protocol (layered protocols make it easier to modify and implement the networking task). Routers operate at Layer 3, the packet layer. The basic routing concept is that of a route. Routes on a network, whether the global Internet or the network within your company, are the path that messages take to reach their destination. But Layer 3 packets are placed inside Layer 2 frames (like Ethernet frames), and a network node that only looks at frames is called a bridge. A switch, as defined in this book, is a bridge that uses frames with special tags called virtual LANs (VLANs), to forward traffic.
Layer 1, the Physical Layer, “spits bits” across a medium directly connecting two devices, called adjacent systems. At each adjacent device, or even mid-span on long links, bits are cleaned up and regenerated by a device called a repeater. Repeaters used to be a big deal, but their operation is pretty much a given today, so we won't dwell on them any further. That's all there is to Layer 1.
However, for the purposes of this chapter about routing, bridging, and switching (and for Chapter 11 on EX switches), Layer 2 and Layer 3 are important, and you need to know the differences between networking at Layer 2 and networking at Layer 3. We explain these differences in the following sections.