28 Chapter 1: LAN Switching Review and Configuring Cisco 2950 LAN Switches

Foundation Summary

The “Foundation Summary” section lists the most important facts from the chapter. Although this section does not list everything that will be on the exam, a well-prepared CCNA candidate should at a minimum know all the details in each Foundation Summary before taking the exam.

Switches basically perform three tasks:

■Learning—The switch learns MAC addresses by examining the source MAC address of each frame the bridge receives. By learning, the switch can make good forwarding choices in the future.

■Forwarding or filtering—The switch decides when to forward a frame or when to filter (not forward) it based on the destination MAC address. The switch looks at the previously learned MAC addresses in an address table to decide where to forward the frames.

■Loop prevention—The switch creates a loop-free environment with other bridges by using the Spanning Tree Protocol (STP). Having physically redundant links helps LAN availability, and STP prevents the switch logic from letting frames loop around the network indefinitely, congesting the LAN.

The following list is a quick review of the basic logic used by a switch:

1.A frame is received.

2.If the destination is a broadcast or multicast, forward on all ports except the port on which the frame was received.

3.If the destination is a unicast, and the address is not in the address table, forward on all ports except the port on which the frame was received.

4.If the destination is a unicast, and the address is in the address table, and if the associated interface is not the interface on which the frame arrived, forward the frame out the one correct port.

5.Otherwise, filter (do not forward) the frame.

Q&A 29

Q&A

As mentioned in the Introduction, you have two choices for review questions. The following questions give you a bigger challenge than the exam because they are open-ended. By reviewing with this more-difficult question format, you can exercise your memory better and prove your conceptual and factual knowledge of the topics covered in this chapter. The answers to these questions are found in Appendix A.

For more practice with exam-like question formats, including multiple-choice questions and those using a router simulator, use the exam engine on the CD.

1.Describe how a switch decides whether it should forward a frame, and tell how it chooses the output interface.

2.How does a switch build its address table?

3.What configuration command causes the switch to require a password from a user at the console? What configuration mode context must you be in? (That is, what command(s) must you enter before this command after entering configuration mode?) List the commands in the order you must enter them while in config mode.

4.What configuration command is used to tell the switch the password that is required at the console? What configuration mode context must you be in? (That is, what command(s) must you enter before this command after entering configuration mode?) List the commands in the order in which you must enter them while in config mode.

5.What command sets the password that is required after you enter the enable command? Is that password encrypted by default?

6.Is the password required at the console the same one that is required when Telnet is used to access a switch?

7.Name two commands used to view the configuration to be used at a 2950 switch’s next reload. Which one is the more-recent addition to IOS?

8.Name two commands used to view the configuration that is currently used in a 2950 switch. Which one is the more-recent addition to IOS?

This chapter covers the following subjects:

■Spanning Tree Protocol

■Rapid Spanning Tree (IEEE 802.1w)

■Spanning Tree Protocol Configuration