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CCNP 642-811 BCMSN Exam Certification Guide - Cisco press

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76 Chapter 3: Switch Operation

Suppose this same command produced no output for the interface and VLAN. What might that mean? Either the host has not sent a frame that the switch can use for learning its location, or something odd is going on. Perhaps, the host is using two network interface cards (NICs) to load balance traffic—one NIC is only receiving traffic while the other is only sending. Therefore, the switch never hears and learns the receiving-only NIC address.

To see the CAM table’s size, use the show mac address-table count command. MAC address totals are shown for each active VLAN on the switch. This can give you a good idea about the size of the CAM table and how many hosts are using the network. Be aware that many MAC addresses can be learned on a switch’s uplink ports.

CAM table entries can be manually cleared, if needed, by using the following EXEC command:

Switch# clear mac address-table dynamic [address mac-address | interface type mod/num | vlan vlan-id]

Frequently, you will need to know where a user with a certain MAC address is connected. In a large network, discerning at which switch and switch port a MAC address can be found might be difficult. Start out at the network’s center, or core, and display the CAM table entry for the MAC address.

Look at the switch port shown in the entry and move to the neighboring switch connected to that port. Then, repeat the CAM table process. Keep moving from switch to switch until you reach the edge of the network where the MAC address connects.

TCAM Operation

The TCAM in a switch is more or less self-sufficient. Access lists are automatically compiled or merged into the TCAM, so there is nothing to configure. The only concept you need to be aware of is how the TCAM resources are being used.

TCAMs have a limited number of usable mask, value pattern, and LOU entries. If access lists grow to be large, or many Layer 4 operations are needed, the TCAM tables and registers can overflow. To see the current TCAM resource usage, use the show tcam counts EXEC command. To see the current TCAM partitioning, you can use the show sdm prefer EXEC command. You can repartition the TCAM with some configuration commands, but that is beyond the scope of this book.

Foundation Summary 77

Foundation Summary

The Foundation Summary is a collection of tables, lists, and other 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 might 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 prep before the exam, the following information is a convenient way to review the day before the exam:

Layer 2 switches learn incoming MAC addresses and record their locations based on the inbound switch ports.

Layer 2 switching information is stored in the Content Addressable Memory (CAM) table. The CAM is consulted to find the outbound switch port when forwarding frames.

Multilayer switching looks at the Layer 2 addresses, along with Layer 3 and 4 address and port information, to forward packets.

Multilayer switching is performed in hardware using the Cisco Express Forwarding (CEF) method.

CEF builds Layer 3 destination information from routing tables and Layer 2 data. This information is stored in hardware as a Forwarding Information Base (FIB) table.

Multilayer switches can make many policy decisions in parallel, using the Ternary Content Addressable Memory (TCAM) contents.

TCAM combines a 134-bit Value, or pattern (made up of addresses, port numbers, or other appropriate fields) with a 134-bit Mask to yield a Result value. The Result instructs the switch hardware how to finish forwarding the packet.

Access lists for security (traditional router ACLs and VLAN ACLs) and QoS ACLs are compiled or merged into TCAM entries. These access lists can then be processed on each packet that passes through the switch, as a single table lookup.

As a packet exits a multilayer switch, it must be rewritten so that its header and checksum values are valid. The fields in the original packet that the switch updates are as follows:

Source MAC address becomes the Layer 3 switch MAC address.

Destination MAC address becomes the next-hop MAC address.

IP TTL value is decremented by one.

IP checksum is recomputed.

Ethernet frame checksum is recomputed.

78 Chapter 3: Switch Operation

Table 3-3 Switching Table Commands


Command Syntax



Set the CAM table aging time.

mac address-table aging-time seconds



Configure a static CAM entry.

mac address-table static mac-address vlan vlan-id interface type





Clear a CAM table entry.

clear mac address-table dynamic [address mac-address |


interface type mod/num | vlan vlan-id]



Set privileged level password.

enable password level 15 password



View the CAM table.

show mac address-table dynamic [address mac-address |


interface type mod/num | vlan vlan-id]



View the CAM table size.

show mac address-table count



View TCAM resource information.

show tcam counts



Q&A 79


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 answer. 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.By default, how long are CAM table entries kept before they are aged out?

2.A TCAM lookup involves which values?

3.How many table lookups are required to find a MAC address in the CAM table?

4.How many table lookups are required to match a packet against an access list that has been compiled into 10 TCAM entries?

5.How many value patterns can a TCAM store for each mask?

6.Can all packets be switched in hardware by a multilayer switch?

7.Multilayer switches must rewrite which portions of an Ethernet frame?

8.If a station only receives Ethernet frames and doesn’t transmit anything, how will a switch learn of its location?

9.What is a TCAM’s main purpose?

10.Why do the TCAM mask and pattern fields consist of so many bits?

11.In a multilayer switch with a TCAM, a longer access list (more ACEs or statements) takes longer to process for each frame. True or false?

12.A multilayer switch receives a packet with a certain destination IP address. Suppose the switch has that IP address in its Layer 3 forwarding table, but no corresponding Layer 2 address. What happens to the packet next?

13.If a multilayer switch can’t support a protocol with CEF, it relies on fallback bridging. Can the switch still route that traffic?

14.To configure a static CAM table entry, the mac address-table static mac-address command is used. Which two other parameters must also be given?

15.As a network administrator, what aspects of a switch TCAM should you be concerned with?

80Chapter 3: Switch Operation

16.What portion of the TCAM is used to evaluate port number comparisons in an access list?

17.Someone has asked you where the host with MAC address 00-10-20-30-40-50 is located. Assuming you already know the switch it is connected to, what command can you use to find it?

18. Complete this command to display the size of the CAM table: show mac



19.What protocol is used to advertise CAM table entries among neighboring switches?

20.Suppose a host uses one MAC address to send frames and another to receive them. In other words, one address will always be the source address sent in frames, and the other is only used as a destination address in incoming frames. Is it possible for that host to communicate with others through a Layer 2 switch? If so, how?

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

Switch Management—This section describes the software operating systems that are available on Cisco Catalyst switches, along with the command-line interface (CLI) that is used for configuration and troubleshooting. In addition, this section covers the basic Catalyst switch configuration and administration commands and techniques for interswitch communication.

Switch File Management—This section explains the various files and file systems used in a Catalyst switch, along with the commands necessary to manage them.

Troubleshooting from the Operating System—This section presents a brief overview of the commands that you can use to verify or troubleshoot basic switch operation.

C H A P T E R 4

Switch Configuration

Chapter 3 covered the topic of switch operation from the ground up. This chapter adds to that by reviewing the Catalyst operating systems—the mechanisms by which you can connect to a switch to configure and monitor how it works. Catalyst file systems are explained, along with the files needed to make a switch functional.

This chapter also covers the configuration steps for switch management. Management functions include the methods used to connect to a switch, and configuring switch identification, user authentication, inter-switch communication, and file management. A brief overview of useful troubleshooting commands is also given.

”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 4-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.

Table 4-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

Foundation Topics Section

Questions Covered in This Section



Switch Management




Switch File Management




Troubleshooting from the Operating






84 Chapter 4: Switch Configuration

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 selfassessment results and might provide you with a false sense of security.

1.Which of the following is an operating system available on Cisco Catalyst 3550, 4500, and 6500 family switches?

a.Catalyst OS




2.Which of the following is not a valid way to connect to a Catalyst switch?



c.async serial


3.Which user interface mode allows the greatest authority for making configuration changes?

a.User EXEC mode

b.Privileged EXEC (enable) mode

c.Telnet mode

d.Root mode

4.To configure a password for Telnet access to a switch, which one of the following must the password be applied to?

a.interface vlan 1

b.line con 0

c.line vty 0 15


”Do I Know This Already?” Quiz 85

5.Which of the following is not required to set up Telnet access to a switch?

a.Password on vty

b.IP address

c.Default gateway or route

d.Enable password

6.Which protocol is used to exchange information between connected Cisco neighbors?





7.Cisco Discovery Protocol is sent over which OSI layer?

a.Layer 1

b.Layer 2

c.Layer 3

d.Layer 4

8.Which Catalyst file system contains the running IOS software image?





9.Which command saves newly made configuration changes so they will be automatically used after the next switch reload?

a.save all

b.copy running-config flash:

c.copy startup-config running-config

d.copy running-config startup-config

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