Understanding 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:

• What Is Spanning Tree and Why Use Spanning Tree— Briefly explains the purpose of the Spanning-Tree Protocol (STP). Explains why some form of loop-prevention protocol is required to prevent broadcast storms and bridge table corruption.

• Four-Step STP Decision Sequence— Describes the process that the Spanning-Tree Protocol uses for all evaluations and calculations.

• Initial STP Convergence— A detailed examination of the three steps that STP uses to initially converge on a loop-free active topology.

• STP States— Explains the five STP states and how the algorithm progresses through each state.

• STP Timers— Discusses the three configurable timers utilized by the Spanning-Tree Protocol.

• The show spantree Command— Provides a detailed explanation of the fields contained in this powerful command. Several useful tips are discussed.

• BPDUs— Provides a detailed discussion of the frames used by bridges and switches to convey STP information. Decodes of actual BPDUs are explained.

• Topology Change Process— Explains how the Topology Change process allows the network to reconverge more quickly after changes in the physical network.

• Setting the Root Bridge— Explains how to manually place Root Bridges in your network for improved stability and performance.

• Per VLAN Spanning Tree— Explains how Cisco supports one instance of the Spanning-Tree Protocol per VLAN. This features allows for extremely flexible designs and is detailed in Chapter 7, "Advanced Spanning Tree."

Most network administrators and designers underestimate the importance of the Spanning-Tree Protocol (STP). As routers became popular in the early 1990s, STP faded into the background as a "less important protocol that just worked." However, with the recent rise of switching technology, Spanning Tree has once again become an important factor that can have a tremendous impact on your network's performance.

In fact, STP often accounts for more than 50 percent of the configuration, troubleshooting, and maintenance headaches in real-world campus networks (especially if they are poorly designed). When I first encountered switching technology, I had the typical "I'm a Layer 3 pro, how hard could this STP stuff be?" mentality. However, I soon learned that STP is a complex protocol that is generally very poorly understood. I found it difficult to locate good Spanning Tree information, especially information about modern implementations of STP. The goal of this chapter (and Chapter 7) is to make your STP journey smooth sailing.

This chapter covers the mechanics of the Spanning-Tree Protocol as it performs its basic loop-prevention duties. To build a baseline knowledge of STP, the chapter begins by answering the questions "What is Spanning Tree?" and "Why do I need

Spanning Tree?" From there, the chapter walks through the Spanning Tree algorithm in detail. In short, this chapter sets the stage for Chapter 7, "Advanced Spanning Tree," where complex topics such as load balancing and minimizing convergence time are presented in detail.

This chapter uses the terms bridge, switch, and Layer 2 switch interchangeably. Although some argue that there are differences between these types of devices, these differences are irrelevant when discussing Spanning Tree. This is particularly true when discussing the STP standards that were written prior to the development of hardware-based switches. For example, you will learn about the Root Bridge concept (don't worry about what it means yet). Although the term Root Switch is becoming more common, I find it awkward when first learning how the Spanning-Tree Protocol functions. However, the term switch is used when discussing particular network designs and deployments because it is rare to deploy a traditional, software-based bridge today.

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