Understanding Linux Network Internals

Chapter 15. Bridging: The Spanning Tree Protocol

We saw in Chapter 14 that transparent bridging represents an easy way to merge LANs, but it can be used only on loop-free topologies. This limitation eliminates the use of transparent bridges on networks where redundant links are used to increase overall availability.

In this chapter, we will see how the Spanning Tree Protocol (STP) manages to make any topology loop free, and therefore allows the network administrator to use topologies with redundant links. In particular, we will see:

• How the distributed algorithm used by STP leads to a loop-free topology by disabling the right redundant links. The loop-free topology selected by STP is a tree (which by definition is loop free). All the traffic between the hosts of the LANs connected by the bridges travels along the links of this tree.

• How STP dynamically updates the topology to cope with configuration changes and bridge or link failures.

• How STP dynamically updates the forwarding database (i.e., addresses learned on the bridge ports) when changes in the topology are detected.

We cannot go into detail on the STP for lack of space. The goal of this chapter is to give you an overview detailed enough to make you comfortable with the description of the kernel implementation of STP discussed in Chapter 16. For a complete discussion of the 802.1D STP and its enhancements, please refer to the IEEE specifications.

The examples provided in this chapter do not function as a guide on how to configure ...