Chapter 4: Layer 2 Switching and the Spanning Tree Protocol (STP)

The CCNP Exam Topics Covered in This Chapter Include the Following:

• Understand the physical, data-link, and network layer technologies used in a multi-layer switched network

• Describe Spanning Tree (STP), and explain the operation of common and per-VLAN STP implementations

• Configure Spanning Tree in both Common Spanning Tree (CST) and per-VLAN modes

In this chapter, we’ll explore the three distinct functions of layer 2 switching: address filtering, forward/filter decision-making, and loop avoidance. We will probe the issue of loop avoidance in depth and discuss how the Spanning Tree Protocol (STP) works to stop network loops from occurring on your layer 2 network.

This chapter continues the discussion of layer 2 switching started in Chapter 1, “The Campus Network.” We will consider the different modes of switching that may be employed, move on to see how network loops occur in a layer 2 network, and then provide an introduction to STP, including the different components of STP and how to configure STP on layer 2 switched networks. It is necessary for networking professionals to have a clear understanding of the STP, so by the end of this chapter, you will know how to use STP to stop network loops, broadcast storms, and multiple frame copies. In Chapter 5, “Using Spanning Tree with VLANs,” we’ll continue discussing STP and provide the more complex and advanced configurations used with it.

It is typical these days to create a network with redundant links; this provides consistent network availability when a network outage occurs on one link. STP provides the necessary loop- avoidance function, but there are several additional features that can be utilized. For example, it is possible to load-balance over the redundant links as well, and VLANs have a special part to play, so we will continue the discussion in Chapter 5.