Understanding Linux Network Internals

Example of Hierarchical Switched L2 Topology

We know that two hosts can be connected to each other with a cross cable; you do not necessarily have to use a device such as a hub or a bridge. You can do the same between bridges and routers. In the examples in this chapter, you will often see such cross-cable links between bridges.

Unlike the scenarios in Chapter 14, where simple two-port bridges link directly to hosts located in the connected LANs, a real bridged network normally has a topology that resembles a tree, where hosts are located only (or mainly) at the leaves' node.

Figure 15-1. Basic terminology

When you have simple scenarios like the ones seen in Chapter 14, you do not usually enable the STP; in fact, most sites could simply use a single flat LAN instead of employing bridges at all. To better understand STP, we need to see what a real-life scenario looks like. Let's take the classic hierarchical bridged and redundant topology in Figure 15-2(a), which is advertised and evangelized by most of the commercial bridge vendors.

The figure leaves out details described later in this chapter, such as the bridge ID and priority, port cost, and priority values, to let you focus on the topology and active links selection. In the subsequent figures in this chapter, we will reuse the symbol definitions in the legend at the bottom of the figure.

At the leaves of the tree (the bottom of the ...