CCNP Exam Prep: Traditional Spanning Tree Protocol

This chapter covers the following topics that you need to master for the CCNP BCMSN exam:

• IEEE 802.1D Overview— This section discusses the original, or more traditional, Spanning Tree Protocol (STP). This protocol is the foundation for the default Catalyst STP and for all the enhancements that are described in Chapters 10, "Spanning Tree Configuration," through 12, "Advanced Spanning Tree Protocol."
• Types of STP— This section discusses other types of STP that might be running on a Catalyst switch—specifically, the Common Spanning Tree, Per-VLAN Spanning Tree (PVST), and PVST+.

Previous chapters covered robust network designs where redundant links are used between switches. Although this increases the network availability, it also opens up the possibility for conditions that would impair the network. In a Layer 2 switched network, preventing bridging loops from forming over redundant paths is important. Spanning Tree Protocol (STP) was designed to monitor and control the Layer 2 network so that a loop-free topology is maintained.

This chapter discusses the theory and operation of the STP. More specifically, the original, or traditional, STP is covered, as defined in IEEE 802.1D. Several chapters explain STP topics in this book. Here is a brief roadmap so you can chart a course:

• Chapter 9, "Traditional Spanning Tree Protocol"— Covers the theory of IEEE 802.1D
• Chapter 10, "Spanning Tree Configuration"— Covers the configuration commands needed for IEEE 802.1D
• Chapter 11, "Protecting the Spanning Tree Protocol Topology"— Covers the features and commands to filter and protect a converged STP topology from conditions that could destabilize it
• Chapter 12, "Advanced Spanning Tree Protocol"— Covers the newer 802.1w and 802.1s enhancements to STP, allowing more scalability and faster convergence

"Do I Know This Already?" Quiz

The purpose of the "Do I Know This Already?" quiz is to help you decide whether you need to read the entire chapter. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.

The 12-question quiz, derived from the major sections in the "Foundation Topics" portion of the chapter, helps you determine how to spend your limited study time.

Table 9-1 outlines the major topics discussed in this chapter and the "Do I Know This Already?" quiz questions that correspond to those topics.

Table 9-1. "Do I Know This Already?" Foundation Topics Section-to-Question Mapping

1. How is a bridging loop best described?

   1. A loop formed between switches for redundancy
   2. A loop formed by the Spanning Tree Protocol
   3. A loop formed between switches where frames circulate endlessly
   4. The round-trip path a frame takes from source to destination

2. Which of these is one of the parameters used to elect a Root Bridge?

   1. Root Path Cost
   2. Path Cost
   3. Bridge Priority
   4. BPDU revision number

3. If all switches in a network are left at their default STP values, which one of the following is not true?

   1. The Root Bridge will be the switch with the lowest MAC address.
   2. The Root Bridge will be the switch with the highest MAC address.
   3. One or more switches will have a Bridge Priority of 32,768.
   4. A secondary Root Bridge will be present on the network.

4. Configuration BPDUs are originated by which of the following?

   1. All switches in the STP domain
   2. Only the Root Bridge switch
   3. Only the switch that detects a topology change
   4. Only the secondary Root Bridge when it takes over

5. Which of these is the single most important design decision to be made in a network running STP?

   1. Removing any redundant links
   2. Making sure all switches run the same version of IEEE 802.1D
   3. Root Bridge placement
   4. Making sure all switches have redundant links

6. What happens to a port that is neither a Root Port nor a Designated Port?

   1. It is available for normal use.
   2. It can be used for load balancing.
   3. It is put into the Blocking state.
   4. It is disabled.

7. What is the maximum number of Root Ports that a Catalyst switch can have?

   1. 1
   2. 2
   3. Unlimited
   4. None

8. What mechanism is used to set STP timer values for all switches in a network?

   1. Configuring the timers on every switch in the network.
   2. Configuring the timers on the Root Bridge switch.
   3. Configuring the timers on both primary and secondary Root Bridge switches.
   4. The timers can't be adjusted.

9. MAC addresses can be placed into the CAM table, but no data can be sent or received if a switch port is in which of the following STP states?

   1. Blocking
   2. Forwarding
   3. Listening
   4. Learning

10. What is the default "hello" time for IEEE 802.1D?

    1. 1 second
    2. 2 seconds
    3. 30 seconds
    4. 60 seconds

11. Which of the following is the Spanning Tree Protocol defined in the IEEE 802.1Q standard?

    1. PVST
    2. CST
    3. EST
    4. MST

12. If a switch has 10 VLANs defined and active, how many instances of STP will run using PVST+ versus CST?

    1. 1 for PVST+, 1 for CST
    2. 1 for PVST+, 10 for CST
    3. 10 for PVST+, 1 for CST
    4. 10 for PVST+, 10 for CST

You can find the answers to the quiz in Appendix A, "Answers to Chapter 'Do I Know This Already?' Quizzes and Q&A Sections." The suggested choices for your next step are as follows:

• 7 or less overall score— Read the entire chapter. This includes the "Foundation Topics," "Foundation Summary," and "Q&A" sections.
• 8–10 overall score— Begin with the "Foundation Summary" section and then follow up with the "Q&A" section at the end of the chapter.
• 11 or more overall score— If you want more review on these topics, skip to the "Foundation Summary" section and then go to the "Q&A" section at the end of the chapter. Otherwise, move to Chapter 10.