CCNP 642-811 BCMSN Exam Certification Guide - Cisco press

256 Chapter 10: Spannning Tree Configuration

Table 10-3 Commands for Displaying Spanning Tree Information

1The PortFast state is not readily accessible but is included in the large amount of information from the show spanningtree command. You can add output filters to include only the lines that reference the Port and the portfast status.

Foundation Summary 257

Foundation Summary

The Foundation Summary is a collection of information that provides a convenient review of many key concepts in this chapter. If you are already comfortable with the topics in this chapter, this summary can help you recall a few details. If you just read this chapter, this review should help solidify some key facts. If you are doing your final preparation before the exam, these tables and figures are a convenient way to review the day before the exam.

Table 10-4 STP Configuration Commands

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Q&A

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 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?

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?

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?

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?

5.If switch B’s Bridge Priority is changed to 10,000, which one will be Root?

6.If switch B’s Bridge Priority is changed to 32,769, which one will be Root?

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

Q&A 259

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 happens?

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

b.The new switch becomes and stays the Root Bridge (Bridge Priority 8192).

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

d.The new switch detects that a Root Bridge already exists and raises its own Bridge Priority to 32,768.

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 becomes the primary and secondary Root Bridge?

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

11.Which STP timer determines how long a port stays in the Listening state? What is its default value?

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

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?

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

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

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?

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.

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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?

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

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

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?

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

■Root Guard—This section discusses how to protect the STP topology against unexpected switches advertising to become the Root Bridge.

■BPDU Guard—This section covers unexpected STP advertisements on switch ports configured for PortFast, where single hosts connect.

■BPDU Skew Detection—This section shows how to detect delayed BPDU reception.

■Loop Guard—This section discusses how to protect the STP topology against the loss of BPDUs from the Root Bridge on a switch port.

■UDLD—This section presents a feature to detect and protect against unidirectional, switch-to-switch links.

■Troubleshooting STP Protection—This section summarizes the commands that diagnose or verify actions that take to protect the topology.

C H A P T E R 11

Protecting the Spanning Tree

Protocol Topology

Achieving and maintaining a loop-free Spanning Tree Protocol (STP) topology revolves around the simple process of sending and receiving bridge protocol data units (BPDUs). Under normal conditions where all switches are playing fairly and according to the rules, a loop-free topology is dynamically determined.

This chapter discusses two basic conditions that can occur to disrupt the loop-free topology (even while STP is running):

■On a port that has not been receiving BPDUs, BPDUs are not expected. When BPDUs suddenly appear for some reason, the STP topology can reconverge to give unexpected results.

■On a port that normally receives BPDUs, BPDUs are always expected. When BPDUs suddenly disappear for some reason, a switch can make incorrect assumptions about the topology and unintentionally create loops.

“Do I Know This Already?” Quiz

The purpose of the “Do I Know This Already?” quiz is to help you decide what parts of this chapter to use. If you intend to read the entire chapter, you do not necessarily need to answer these questions now.

The 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 11-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.

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Table 11-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

CAUTION The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark this question wrong. Giving yourself credit for an answer you correctly guess skews your self-assessment results and might give you a false sense of security.

1.Why is it important to protect the placement of the Root Bridge?

a.To keep two Root Bridges from becoming active

b.To keep the STP topology stable

c.So all hosts have the correct gateway

d.So the Root Bridge can have complete knowledge of the STP topology

2.Which of the following features protects a switch port from accepting superior BPDUs?

a.STP loop guard

b.STP BPDU guard

c.STP root guard

d.UDLD

3.Which of the following commands can you use to enable STP root guard on a switch port?

a.spanning-tree root guard

b.spanning-tree root-guard

c.spanning-tree guard root

d.spanning-tree rootguard enable

“Do I Know This Already?” Quiz 265

4.Where should the STP root guard feature be enabled on a switch?

a.All ports

b.Only ports where the Root Bridge should never appear

c.Only ports where the Root Bridge should be located

d.Only ports with PortFast enabled

5.Which of the following features protects a switch port from accepting BPDUs when PortFast is enabled?

a.STP loop guard

b.STP BPDU guard

c.STP root guard

d.UDLD

6.Which of the following features can you use to identify when BPDUs are delayed as they pass through the topology?

a.UDLD

b.BPDU guard

c.BPDU authentication

d.BPDU skew detection

7.To maintain a loop-free STP topology, which one of the following should a switch uplink be protected against?

a.A sudden loss of BPDUs

b.Too many BPDUs

c.The wrong version of BPDUs

d.BPDUs relayed from the Root Bridge

8.Which of the following commands can enable STP loop guard on a switch port?

a.spanning-tree loop guard

b.spanning-tree guard loop

c.spanning-tree loop-guard

d.spanning-tree loopguard enable