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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.

Chapter 9 Answers

"Do I Know This Already?"

  1. c

  2. c

  3. b

  4. b

  5. c

  6. c

  7. a

  8. b

  9. d

  10. b

  11. b

  12. c


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

    A bridging loop is a path through a bridged or switched network that provides connectivity in an endless loop. Unknown unicast, broadcast, or multicast frames introduced into the loop are propagated by each switch, causing the frames to circulate around and around the loop. Network bandwidth and CPU resources can be absorbed completely by the increasing amount of broadcast traffic. Breaking the loop connectivity can end bridging loops.

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

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

    d, c, a, b

  3. Choose two types of STP messages used to communicate between bridges:

    1. Advertisement BPDU
    2. Configuration BPDU
    3. ACK BPDU
    4. TCN BPDU

    b, d

  4. What criteria are used to select the following?

    1. Root Bridge
    2. Root Port
    3. Designated Port
    4. Redundant (or secondary) Root Bridges
    1. Lowest Bridge ID (Bridge priority, MAC address)
    2. Lowest Root Path Cost
    3. Lowest Root Path Cost on a shared segment
    4. Next-to-lowest Bridge ID

    If a tie occurs, these parameters are used to decide:

    1. Lowest Bridge ID
    2. Lowest Root Path Cost
    3. Lowest Sender Bridge ID
    4. Lowest Sender Port ID
  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

    Catalyst C will become the primary Root because of its lower Bridge Priority value. (Bridge Priority has a greater weight on the election than a lower MAC address.) The secondary Root will be Catalyst B; both A and B have the next-lowest Bridge Priorities, but B also has a lower MAC address.

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

    A topology change occurs when a port moves to the Forwarding state, or from Forwarding or Learning to the Blocking state. During a topology change, addresses are aged out in Forward Delay seconds, whereas active stations are not aged out of the bridging table. The STP is not recomputed; TCN BPDUs are sent throughout the network, notifying other switches of the topology change. Only the port where the topology change is occurring is affected, by moving through the STP states.

  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?

    The new switch will begin life by advertising itself as the Root Bridge, thinking it is the only bridge on the network. Because it has a lower Bridge ID than the current Root, it will win the election after the BPDUs converge and when all switches have a knowledge of the new, better choice.

  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?

    The STP doesn't allow more than one Root Port per switch (bridge). Because of this, both ports cannot become Root Ports. Only the port with the lowest Root Path Cost (or one of the successive STP tie-breaker decisions) will become the Root Port.

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

    The Root Path cost is a cumulative value that is incremented as Configuration BPDUs are passed from switch to switch. A switch adds the Port Cost of the local port to the current Root Path Cost value as a BPDU is received.

  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.)

    By definition, all ports on the Root Bridge are Designated Ports because they are in the closest possible location to the Root Bridge. Therefore, those ports never can be put into the Blocking state. The only exception to this is if two of the Root Bridge switch's ports are connected, a situation that could but shouldn't occur.

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

    Port Cost

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

    Turn off the switch or unplug a cable on a port that is part of the loop. Turning off the switch is obviously a drastic measure, but it does help to clear the loop. Any method might be used, as long as the loop manually is broken or disconnected. In some cases, the traffic volume caused by the loop can overwhelm the switch CPU. If that happens, you won't be able to connect to the switch CLI to shut down an interface or reload the switch.

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

    When the switch that is sending the BPDU is also the Root Bridge

  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.

    b, d

  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.

    Switch A will have the Designated Port. The STP tie-breaking sequence must be used for the decision. The first relevant decision is that of the lowest Root Path Cost, advertised by switch A. If both switches advertised an identical Root Path Cost, the lowest Sender Bridge ID (that of switch B) would be used.

  16. Using the default STP timers, how long does it take for a port to move from the Blocking state to the Forwarding state?

    30 seconds

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

    Shorten their bridge table aging times

  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+?

    PVST+ interoperates with CST 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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