Chapter 7. Advanced Spanning Tree

The authors would like to thank Radia Perlman for graciously contributing her time to review the material in this chapter.

This chapter covers the following key topics:

• Typical Campus Design: A Baseline Network Introduces a baseline network for use throughout most of the chapter.

• STP Behavior in the Baseline Network: A Spanning Tree Review Reviews the concepts introduced in Chapter 6, "Understanding Spanning Tree," while also introducing some advanced STP theory.

• Spanning Tree Load Balancing Master this potentially confusing and poorly documented feature that can double available bandwidth for free. Four Spanning Tree load balancing techniques are discussed in detail.

• Fast STP Convergence Seven techniques that can be used to improve on the default STP failover behavior of 30 50 seconds.

• Useful STP Display Commands Several commands that can be extremely useful for understanding and troubleshooting STP's behavior in your network.

• Per-VLAN Spanning Tree Plus (PVST+) An important feature introduced by Cisco, PVST+ allows interoperability between traditional per-VLAN Spanning Tree (PVST) Catalysts and devices that support 802.1Q.

• Disabling STP Explains how to disable Spanning Tree on Catalyst devices. Also considers reasons why this might be done and why it shouldn't be done.

• Tips and Tricks: Mastering STP A condensed listing of Spanning Tree advice to help you avoid STP problems in your network.

Chapter 6, "Understanding Spanning Tree," constructed a solid foundation of Spanning Tree knowledge. This chapter builds on that base by looking at a variety of advanced issues related to the Spanning-Tree Protocol (STP). After a quick review of some key points from Chapter 6, the discussion examines the important issue of load balancing. The techniques discussed in this section might allow you to double your available bandwidth in a redundant configuration without buying any new equipment! The chapter then discusses methods for improving Spanning Tree convergence and failover times. After a detailed look at some important show commands and new features, the chapter concludes with a section called "Tips and Tricks: Mastering STP." This information is designed to be a compendium of useful Spanning Tree information and best practices.

If you skipped Chapter 6, "Understanding Spanning Tree," consider that some of the techniques discussed in this chapter are not intuitively obvious. For example, a good understanding of STP basics is required to comprehend Spanning Tree load balancing. At the very least, I recommend that you do the exercises located at the end of Chapter 6. If these seem straightforward, by all means, continue. However, if they seem strange and mysterious, you might want to go back and read Chapter 6.

Caution

As with Chapter 6, the examples in this chapter are designed to illustrate the intricacies of the Spanning-Tree Protocol, not good design practices. For more information on Spanning Tree and campus design principles, please refer to Chapter 11, "Layer 3 Switching," Chapter 14, "Campus Design Models," Chapter 15, "Campus Design Implementation," and Chapter 17, "Case Studies: Implementing Switches."