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Switch Operation for the CCNP BCMSN Exam

Chapter Description

This sample chapter covers the following topics that you need to master for the CCNP BCMSN exam: Layer 2 Switch Operation, Multilayer Switch Operation, Tables Used in Switching, and Troubleshooting Switching Tables.

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

  • Layer 2 Switch Operation—This section describes the functionality of a switch that forwards Ethernet frames.

  • Multilayer Switch Operation—This section describes the mechanisms that forward packets at OSI Layers 3 and 4.

  • Tables Used in Switching—This section explains how tables of information and computation are used to make switching decisions. Coverage focuses on the Content Addressable Memory table, involved in Layer 2 forwarding, and the Ternary Content Addressable Memory, used in Layers 2 through 4 packet-handling decisions.

  • Troubleshooting Switching Tables—This section reviews the Catalyst commands that you can use to monitor the switching tables and memory. These commands can be useful when troubleshooting or tracing the sources of data or problems in a switched network.

To have a good understanding of the many features that you can configure on a Catalyst switch, you should first understand the fundamentals of the switching function itself.

This chapter serves as a primer, describing how an Ethernet switch works. It presents Layer 2 forwarding, along with the hardware functions that make forwarding possible. Multilayer switching is also explained. A considerable portion of the chapter deals with the memory architecture that performs switching at Layers 3 and 4 both flexibly and efficiently. This chapter also provides a brief overview of useful switching table management commands.

"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 already intend to read the entire chapter, you do not necessarily need to answer these questions now.

The 12-question 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 3-1 outlines the major topics discussed in this chapter and the "Do I Know This Already?" quiz questions that correspond to those topics.

Table 3-1 "Do I Know This Already?" Foundation Topics Section-to-Question Mapping

Foundation Topics Section

Questions Covered in This Section

Layer 2 Switch Operation


Multilayer Switch Operation


Switching Tables


Troubleshooting Switching Tables



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 provide you with a false sense of security.

  1. Which of these performs transparent bridging?

    1. Ethernet hub

    2. Layer 2 switch

    3. Layer 3 switch

    4. Router

  2. When a PC is connected to a Layer 2 switch port, how far does the collision domain spread?

    1. No collision domain exists.

    2. One switch port.

    3. One VLAN.

    4. All ports on the switch.

  3. What information is used to forward frames in a Layer 2 switch?

    1. Source MAC address

    2. Destination MAC address

    3. Source switch port

    4. IP addresses

  4. What does a switch do if a MAC address can't be found in the CAM table?

    1. The frame is forwarded to the default port.

    2. The switch generates an ARP request for the address.

    3. The switch floods the frame out all ports (except the receiving port).

    4. The switch drops the frame.

  5. In the Catalyst 6500, frames can be filtered with access lists for security and QoS purposes. This filtering occurs according to which of the following?

    1. Before a CAM table lookup

    2. After a CAM table lookup

    3. Simultaneously with a CAM table lookup

    4. According to how the access lists are configured

  6. Access list contents can be merged into which of the following?

    1. A CAM table

    2. A TCAM table

    3. A FIB table

    4. An ARP table

  7. Multilayer switches using CEF are based on which of these techniques?

    1. Route caching

    2. Netflow switching

    3. Topology-based switching

    4. Demand-based switching

  8. Which answer describes multilayer switching with CEF?

    1. The first packet is routed, and then the flow is cached.

    2. The switch supervisor CPU forwards each packet.

    3. The switching hardware learns station addresses and builds a routing database.

    4. A single database of routing information is built for the switching hardware.

  9. In a switch, frames are placed in which buffer after forwarding decisions are made?

    1. Ingress queues

    2. Egress queues

    3. CAM table

    4. TCAM

  10. What size are the mask and pattern fields in a TCAM entry?

    1. 64 bits

    2. 128 bits

    3. 134 bits

    4. 168 bits

  11. Access list rules are compiled as TCAM entries. When a packet is matched against an access list, in what order are the TCAM entries evaluated?

    1. Sequentially in the order of the original access list.

    2. Numerically by the access list number.

    3. Alphabetically by the access list name.

    4. All entries are evaluated in parallel.

  12. Which Catalyst 3550 command can you use to display the addresses in the CAM table?

    1. show cam

    2. show mac address-table

    3. show mac

    4. show cam address-table

You can find the answers to the quiz in Appendix A, "Answers to Chapter 'Do I Know This Already?' Quizzes and Q & A Sections." The suggested choices for your next step are as follows:

  • 7 or less overall score—Read the entire chapter. This includes the "Foundation Topics," "Foundation Summary," and the "Q&A" section.

  • 8–10 overall score—Begin with the "Foundation Summary" section and then follow up with the "Q&A" section at the end of the chapter.

  • 11 or more overall score—If you want more review on these topics, skip to the "Foundation Summary" section and then go to the "Q&A" section at the end of the chapter. Otherwise, move on to Chapter 4, "Switch Configuration."

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