CCNP 642-811 BCMSN Exam Certification Guide - Cisco press

546Appendix A: Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections

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

a.Advertisement BPDU

b.Configuration BPDU

c.ACK BPDU

d.TCN BPDU

Answer: b, d

4.What criteria are used to select the following?

a.Root Bridge

b.Root Port

c.Designated Port

d.Redundant (or secondary) Root Bridges

Answers:

a.Lowest Bridge ID (Bridge priority, MAC address)

b.Lowest Root Path Cost

c.Lowest Root Path Cost on a shared segment

d.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 will become the Root Bridge, given the information in the table below? Which switch will become the secondary Root Bridge if the Root Bridge fails?

Chapter 9 547

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 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 all switches have a knowledge of the new, better choice.

8.Suppose 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 its local port’s Port Cost to the current Root Path Cost value as a BPDU is received.

548Appendix A: Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections

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 together 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 can never be put into the Blocking state. The only exception to this is if two of the Root Bridge switch’s ports are connected together—a situation that could, but shouldn’t ever, 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 the switch off or unplug a cable on a port that is part of the loop.

Turning the switch off is obviously a drastic measure but does help to clear the loop. Any method might be used, as long as the loop is manually 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?

a.It is a value sent by the Root Bridge that cannot be changed along the way.

b.It is incremented as a switch receives a BPDU.

c.It is incremented as a switch sends a BPDU.

d.It is incremented by the Path Cost of a port.

Answer: B, D

Chapter 9 549

15.Suppose two switches are connected by a common link. Each must decide which one will have the Designated Port on the link. Which switch will take on this role, if these STP advertisements occur?

•The link is on switch A’s port number 12 and on switch B’s port number 5.

•Switch A has a Bridge ID of 32,768:0000.1111.2222, and switch B has 8192:0000.5555.6666.

•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 VLAN(s) does the CST form of STP run?

a.VLAN 1

b.All active VLANs

c.All VLANs (active or inactive)

d.The native VLAN

Answer: d

19.What is the major difference between PVST and PVST+? PVST+ interoperates with CST and PVST.

550Appendix A: Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections

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

a.When neither has a better Root Path Cost.

b.When the switches are actually the primary and secondary Root Bridges.

c.When one switch has its port in the Blocking state.

d.Never; this can’t happen.

Answer: d

Chapter 10

“Do I Know This Already?”

1.c

2.c

3.d

4.c

5.c

6.c

7.b

8.d

9.c

10.a

11.a

12.d

Q&A

1.What commands can configure a Catalyst 4500 switch as the Root Bridge on VLAN 10, assuming that the other switches are using the default STP values?

spanning-tree vlan 10 root primary

Chapter 10 551

2.Using your Root Bridge answer from Question 1, what commands can configure a Catalyst 3550 switch as a secondary or backup Root Bridge on VLAN 10?

spanning-tree vlan 10 root secondary

3.Which of the following switches will become the Root Bridge, given the information in the following table? Which switch will become the secondary Root Bridge if the Root Bridge fails?

The Root Bridge will be Catalyst C because its Bridge Priority has the lowest value. The Bridge Priority is more significant because it is stored in the upper bits of the Bridge ID field. If Catalyst C fails in its duty as Root Bridge, Catalyst B will take over as the secondary Root Bridge. Because Catalyst B has the default Bridge Priority (32,768), along with another switch, the lowest MAC address will be the deciding factor.

Questions 4 through 7 are based on a network that contains two switches, Catalyst A and B. Their Bridge Priorities and MAC addresses are 32,768:0000.aaaa.aaaa and 32,768:0000.bbbb.bbbb, respectively.

4.Which switch will become the Root Bridge?

Catalyst A; the Bridge Priorities are equal, so the lowest MAC address is the deciding factor.

5.If switch B’s Bridge Priority is changed to 10,000, which one will be Root? Catalyst B will become the new Root Bridge because its new priority is the lowest.

6.If switch B’s Bridge Priority is changed to 32,769, which one will be Root? Catalyst A will become the Root again because B’s priority is slightly higher.

552Appendix A: Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections

7.If switch C is introduced with 40000:0000.0000.cccc, which will be the secondary Root?

Catalyst B was previously the secondary root, with the second-highest Bridge Priority. It will remain the secondary root because Catalyst C has a higher Bridge Priority.

8.Suppose a switch is configured with the spanning-tree vlan 10 root primary command. Then another switch is connected to the network. The new switch has a Bridge Priority of 8192. Which one of the following will happen?

a.When the new switch advertises itself, the original Root Bridge will detect it and lower its Bridge Priority to 4096 less than the new switch.

b.The new switch will become and stay the Root Bridge (Bridge Priority 8192).

c.No change; both switches keep their current Bridge Priorities.

d.The new switch will detect that a Root Bridge already exists and raise its own Bridge Priority to 32,768.

Answer: b

9.Three switches in a network have the following Bridge Priorities: 32,768, 16,384, and 8192. If a fourth switch is configured with spanning-tree vlan 1 root secondary, what is the Bridge Priority of the switches that will become the primary and secondary Root Bridge?

Primary root: 8192, Secondary root: 16,384

(The switch configured with the root secondary keywords can’t detect any other potential secondary Roots, so it can only set its priority to 28,672.)

10.What STP timer values can be automatically modified by setting the network diameter? Hello timer

Forward Delay timer Max age timer

11.Which STP timer determines how long a port stays in the Listening state? What is its default value? The Forward Delay timer; default 15 seconds

12.What is the purpose of the Max Age timer?

It sets the length of time received BPDUs are held if a neighboring switch is not heard from on a nondesignated port. After the Max Age timer expires, the BPDU for the neighbor is flushed and that port enters the Listening state, eventually becoming the new Designated Port on the segment.

Chapter 10 553

13.Three switches are connected to each other, forming a triangle shape. STP prevents a loop from forming. What is the most accurate value that could be used for the network diameter?

Answer: 3

14.Which of the following will not benefit from STP UplinkFast?

a.An access layer switch with one uplink port

b.An access layer switch with two uplink ports

c.An access layer switch with three uplink ports

d.An access layer switch with four uplink ports

Answer: a

15.What command can enable the STP PortFast feature on a switch? What configuration mode must you enter first?

spanning-tree portfast, in the interface configuration mode

16.What happens if the STP Hello Time is decreased to 1 second in an effort to speed up STP convergence? What happens if the Hello Time is increased to 10 seconds?

Setting the Hello Timer to 1 second doubles the amount of Configuration BPDUs that a switch sends, as compared to the default 2 second timer. While this does share BPDU information more often, it doesn’t help the long convergence delay when a port comes up. The significant delays come from the Forward Delay timer, which is used to move a port through the Listening and Learning states. By default, this process takes 30 seconds and is unaffected by the Hello Timer.

17.What switch command can safely adjust the STP timers on the Root Bridge in VLAN 7? Assume that the network consists of Catalyst A, B, and C, all connected to each other in a triangle fashion.

Because the three switches form a triangle loop, one link will eventually be placed in the Blocking state. Therefore, the maximum distance across the network is 3 switch hops. This value can be used to define the network diameter to safely adjust the STP timers for faster convergence:

spanning-tree vlan 7 root primary diameter 3

554 Appendix A: Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections

For questions 18 and 19, refer to the following output:

18.What is the Bridge ID for the current Root Bridge? Is the switch that produced this output the actual Root Bridge?

The Root Bridge ID is 8000:00d0.0457.3831. The local switch is not the Root Bridge, as its Bridge ID (32818:0009.b7ee.9800) is different from the Root.

19.What is the Path Cost of interface FastEthernet 0/4, and why is it different from the others?

The Path Cost is 100. This is because that interface is currently operating at 10Mbps (STP cost 100), whereas the others are operating at 100Mbps (STP cost 19).

20.Why does the column marked “Designated Bridge ID” have the same value for every switch port?

Each of the switch ports shown has won the election to become the Designated Port for its local segment. Each Designated Port must identify its own Bridge ID, which is 32818:0009.b7ee.9800. Naturally, the Bridge ID is the same for all ports on VLAN 50.

Q&A 555

21.Suppose you need to troubleshoot your Spanning Tree topology and operation. What commands and information can you use on a switch to find information about the current STP topology in VLAN 39?

The show spanning-tree vlan 39 root command will display the current Root Bridge and the Root Port for VLAN 39. The show spanning-tree vlan 39 brief command will show a listing of every switch port on VLAN 39, along with its Path Cost and STP state. The Designated Bridge ID is also shown on every switch port segment. For a quick summary of the total number of ports participating in each active VLAN, use the show spanning-tree summary command.

Chapter 11

“Do I Know This Already?”

1.b

2.c

3.c

4.b

5.b

6.d

7.a

8.b

9.b

10.c

11.c

12.c

Q&A

1.Why would a unidirectional link be bad?

Switches must exchange BPDUs in both directions across a link. If one side of the link is disrupted, and the switches think the link is still operational, one of the switches will not receive BPDUs. If that switch had its end of the link in the blocking state to prevent a bridging loop, the absence of BPDUs will cause it to promote the link toward the forwarding state. At that point, the loop will form.