C H A P T E R 2

Spanning Tree Protocol

When LAN designs require multiple switches, most network engineers include redundant Ethernet segments between the switches. The goal is simple. The switches might fail, and cables might be cut or unplugged, but if redundant switches and cables are installed, the network service might still be available for most users.

LAN designs with redundant links introduce the possibility that frames might loop around the network forever. These looping frames would cause network performance problems. Therefore, LANs use Spanning Tree Protocol (STP), which allows the redundant LAN links to be used while preventing frames from looping around the LAN indefinitely through those redundant links. This chapter covers STP, along with a few configuration commands used to tune how STP behaves.

This chapter covers the details of STP, plus a newer variation called Rapid Spanning Tree Protocol (RSTP). The end of the chapter covers STP configuration on 2950 series switches.

“Do I Know This Already?” Quiz

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

The ten-question quiz, derived from the major sections in the “Foundation Topics” section, helps you determine how to spend your limited study time.

Table 2-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.

Table 2-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

32 Chapter 2: Spanning Tree Protocol

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

1.Which of the following are the port states when STP has completed convergence?

a.Blocking

b.Forwarding

c.Listening

d.Learning

e.Discarding

f.Disabled

2.Which of the following are transitory port states used only during the process of STP convergence?

a.Blocking

b.Forwarding

c.Listening

d.Learning

e.Discarding

f.Disabled

3.Which of the following bridge IDs would win election as root, assuming that the switches with these bridge IDs were in the same network?

a.32768:0200.1111.1111

b.32768:0200.2222.2222

c.200:0200.1111.1111

d.200:0200.2222.2222

e.40,000:0200.1111.1111

f.40,000:0200.2222.2222

“Do I Know This Already?” Quiz 33

4.Which of the following facts determines how often a root bridge or switch sends a BPDU message?

a.The hello interval configured on a nonroot switch

b.The hello interval configured on the root switch

c.It is always every 2 seconds.

d.The switch reacts to BPDUs received from the root switch by sending another BPDU 2 seconds after receiving the root BPDU.

5.What feature causes an interface to be placed in forwarding state as soon as the interface is physically active?

a.STP

b.RSTP

c.Root Guard

d.802.1w

e.PortFast

f.EtherChannel

g.Trunking

h.802.1q

6.What feature combines multiple parallel Ethernet links between two switches so that traffic is balanced across the links, and so that STP treats all links as one link?

a.STP

b.RSTP

c.Root Guard

d.802.1w

e.PortFast

f.EtherChannel

g.Trunking

h.802.1q

34Chapter 2: Spanning Tree Protocol

7.What name represents the improved STP standard that lowers convergence time?

a.STP

b.RSTP

c.Root Guard

d.802.1w

e.PortFast

f.Trunking

g.802.1q

8.Which of the following RSTP port roles have the same name as a similar role in STP?

a.Blocking

b.Forwarding

c.Listening

d.Learning

e.Discarding

f.Disabled

9.On a 2950 switch, what command lets you change the value of the bridge ID without having to configure a specific value for any part of the bridge ID?

a.spanning-tree bridge-id

b.spanning-tree root

c.spanning-tree priority

d.set priority

10.What command lists spanning-tree status information on 2950 series switches?

a.show spanning-tree

b.show stp

c.show spantree

d.show span-tree

e.debug span

“Do I Know This Already?” Quiz 35

The answers to the “Do I Know This Already?” quiz appear in Appendix A. The suggested choices for your next step are as follows:

■8 or less overall score—Read the entire chapter. This includes the “Foundation Topics,” “Foundation Summary,” and “Q&A” sections.

■9 or 10 overall score—If you want more review on these topics, skip to the “Foundation Summary” section and then go to the “Q&A” section. Otherwise, move to the next chapter.