Understanding Linux Network Internals

Chapter 14. Bridging: Concepts

In this first chapter on bridging, we will see what a bridge device is, how it is used, and what limitations it comes with. In particular, I’ll describe transparent bridging, address learning, and the use of the so-called forwarding database. I’ll conclude the chapter with an explanation of why bridges cannot be used on loop topologies and I will introduce the next chapter, where we will see how the Spanning Tree Protocol (STP) can address this limitation. Other forms of bridging are available, but they are rarely used and not implemented in the Linux kernel.

The network topologies used in this chapter do not necessarily represent real case scenarios; they are selected based only on didactic principles.

Repeaters, Bridges, and Routers

Before introducing bridging, I will clarify the distinction between different network devices that forward packets: repeaters, bridges, and routers. The differences are illustrated in Figure 14-1:

• A repeater is a device, typically equipped with two ports, that simply copies what it receives on one port to the other, and vice versa. It copies data bit by bit; it does not have any knowledge of protocols, and therefore cannot distinguish among different frames or packets. Repeaters are rarely used nowadays, because bridges have become pretty affordable and provide better capabilities that justify the cost difference. Multiport repeaters are called hubs.

• Unlike a repeater, a bridge understands link layer protocols and therefore ...