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CCNP Exam Prep: Traditional Spanning Tree Protocol

Chapter Description

This chapter discusses the theory and operation of the STP with an eye toward passing the CCNP BCMSN Exam. More specifically, the original, or traditional, STP is covered, as defined in IEEE 802.1D.


The questions and scenarios in this book are more difficult than what you should experience on the actual exam. The questions do not attempt to cover more breadth or depth than the exam; however, they are designed to make sure that you know the answers. Rather than allowing you to derive the answers from clues hidden inside the questions themselves, the questions challenge your understanding and recall of the subject. Hopefully, these questions will help limit the number of exam questions on which you narrow your choices to two options and then guess.

You can find the answers to these questions in Appendix A.

  1. What is a bridging loop? Why is it bad?

  2. Put the following STP port states in chronological order:

    1. Learning
    2. Forwarding
    3. Listening
    4. Blocking
  3. Choose two types of STP messages used to communicate between bridges:

    1. Advertisement BPDU
    2. Configuration BPDU
    3. ACK BPDU
    4. TCN BPDU
  4. What criteria are used to select the following?

    1. Root Bridge
    2. Root Port
    3. Designated Port
    4. Redundant (or secondary) Root Bridges
  5. Which of the following switches becomes the Root Bridge, given the information in the following table? Which switch becomes the secondary Root Bridge if the Root Bridge fails?

    Switch Name

    Bridge Priority

    MAC Address

    Port Costs

    Catalyst A



    All are 19

    Catalyst B



    All are 4

    Catalyst C



    All are 19

    Catalyst D



    All are 19

  6. What conditions cause an STP topology change? What effect does this have on STP and the network?

  7. A Root Bridge has been elected in a switched network. Suppose that a new switch is installed with a lower Bridge ID than the existing Root Bridge. What will happen?

  8. Suppose that a switch receives Configuration BPDUs on two of its ports. Both ports are assigned to the same VLAN. Each of the BPDUs announces Catalyst A as the Root Bridge. Can the switch use both of these ports as Root Ports? Why?

  9. How is the Root Path Cost calculated for a switch port?

  10. What conditions can cause ports on a network's Root Bridge to move into the Blocking state? (Assume that all switch connections are to other switches. No crossover cables are used to connect two ports on the same switch.)

  11. What parameters can be tuned to influence the selection of a port as a Root or Designated Port?

  12. After a bridging loop forms, how can you stop the endless flow of traffic?

  13. In a BPDU, when can the Root Bridge ID have the same value as the Sender Bridge ID?

  14. Which of these is true about the Root Path Cost?

    1. It is a value sent by the Root Bridge that cannot be changed along the way.
    2. It is incremented as a switch receives a BPDU.
    3. It is incremented as a switch sends a BPDU.
    4. It is incremented by the Path Cost of a port.
  15. Suppose that two switches are connected by a common link. Each must decide which one will have the Designated Port on the link. Which switch takes on this role if these STP advertisements occur?

    1. The link is on switch A's port number 12 and on switch B's port number 5.
    2. Switch A has a Bridge ID of 32,768:0000.1111.2222, and switch B has 8192:0000.5555.6666.
    3. Switch A advertises a Root Path Cost of 8, while B advertises 12.
  16. Using the default STP timers, how long does it take for a port to move from the Blocking state to the Forwarding state?

  17. If the Root Bridge sets the Topology Change flag in the BPDU, what must the other switches in the network do?

  18. Over what VLANs does the CST form of STP run?

    1. VLAN 1
    2. All active VLANs
    3. All VLANs (active or inactive)
    4. The native VLAN
  19. What is the major difference between PVST and PVST+?

  20. Two switches are connected by a common active link. When might neither switch have a Designated Port on the link?

    1. When neither has a better Root Path Cost.
    2. When the switches are actually the primary and secondary Root Bridges.
    3. When one switch has its port in the Blocking state.
    4. Never; this can't happen.
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