CCNA Guide to Cisco Networking Fundamentals, Fourth Edition 12-1
Chapter 12
Basic Switching and Switch Configuration
At a Glance
Instructor’s Manual Table of Contents
• Overview
• Objectives
• Teaching Tips
• Quick Quizzes
• Class Discussion Topics
• Additional Projects
• Additional Resources
• Key Terms
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition 12-2
Lecture Notes
Overview
Up to this point, the focus of this book has been on routing with Cisco routers. In this
chapter and the next, the focus moves to a different piece of equipment: the Cisco layer 2
switch. The discussion covers Ethernet operations, performance and improvement methods,
Fast Ethernet, Gigabit Ethernet, and half- and full-duplex communication modes. Your
students will also learn about Cisco layer 2 switching technologies, terminology, and basic
configurations.
Chapter Objectives
• Explain the technology and media access control method for Ethernet networks
• Explain network segmentation and basic traffic management concepts
• Explain basic switching concepts and the operation of Cisco switches
• Perform and verify switch configuration tasks
• Implement basic switch security
Teaching Tips
Ethernet Operations
1. Define Ethernet as a network access method (or media access method) originated by the
University of Hawaii, later adopted by Xerox Corporation and standardized as IEEE
2. Mention that Ethernet is the most pervasive network access method in use, and is the
most commonly implemented media access method in new LANs.
CSMA/CD
1. Define Carrier Sense Multiple Access with Collision Detection (CSMA/CD) as Ethernet
contention method.
2. Explain that any station connected to a network can transmit anytime a transmission is
3. Explain that two stations could listen to the wire simultaneously and not sense a carrier
4. Mention that once a collision is detected, the first station to detect the collision
5. Define a collision domain as the physical area in which a packet collision might occur.
7. Explain that stations on a network broadcast packets to other stations to make their
8. Mention that one simple way to reduce broadcast traffic is to reduce the number of
services on your network, and to limit the number of protocols in use on your network.
9. Define broadcast storm as a sudden rush of network transmissions that causes all other
10. Mention that one of the most common causes of broadcast storms is a network loop.
Teaching
Tip
Software faults with network card drivers or computer operating systems can
also cause broadcast storms. You can use a protocol analyzer to locate the device
causing the broadcast storm. Once the device is identified, you can correct the
configuration error or apply an appropriate software driver update to correct the
problem.
Latency
1. Define latency, or propagation delay, as the length of time that is required to forward,
3. Explain that the bit time refers to the amount of time required to transmit one data bit on
4. Define slot time (512 bit times) as an important specification that limits the physical
size of each Ethernet collision domain. Slot time specifies that all collisions should be
detected from anywhere in a network in less time than is required to place a 64-byte
frame on the network.
Teaching
Tip
To maintain the same slot time on expanded Ethernet networks the IEEE
instituted the 5-4-3 rule, which limits collision domains to five segments of wire,
four repeaters and/or hubs, and three populated hubs between any two stations on
a 10-Mbps network.
Ethernet Errors
1. Describe the following frame size errors:
2. Explain that a frame check sequence (FCS) error indicates that bits of a frame were
corrupted during transmission. It can be caused by any of the previously listed errors.
3. Explain that collisions are a fact of life on an Ethernet network. Reducing the number of
5. Explain that Fast Ethernet uses the same CSMA/CD as common 10BaseT Ethernet, but
provides ten times the data transmission rate—100 Mbps. It is defined under the IEEE
6. Describe the following Fast Ethernet implementations:
a. 100Base-TX
Gigabit Ethernet
1. Mention that recent advances in technology have allowed us to reach even higher
speeds than those of Fast Ethernet.
2. Describe the following Gigabit Ethernet implementations:
a. 1000Base-TX (802.3ab)
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Half- and Full-Duplex Communications
1. Explain that in half-duplex communications, devices can send and receive signals, but
3. Describe the benefits of using full-duplex, including the following:
a. Time is not wasted retransmitting frames because collisions do not occur.
b. The full bandwidth is available in both directions because the send and receive
functions are separate.
c. Stations do not have to wait until other stations complete their transmissions
because only one transmitter is used for each twisted pair.
Teaching
Tip
Networking tools, like the Fluke One Touch, can determine whether your switch
ports are set for full or half duplex. Some operating system tools, such as mii-
diag in Linux, allow you to verify full- or half-duplex operations. Client network
cards can be configured through the system BIOS, operating system, or
manufacturer-provided software or hardware.
4. Explain that on a Cisco Catalyst 2950 switch, you can set the duplex capabilities port-
by-port. The four different duplex options are:
a. Auto
A Review of LAN Segmentation
1. Explain that you can improve the performance of your Ethernet network by reducing
the number of stations per collision domain. Typically, network administrators
implement bridges, switches, or routers to segment the network and divide the collision
domains.
Segmenting with Bridges
1. Explain that a bridge segments a network by filtering traffic at the Data Link layer. It
2. Mention that bridges learn MAC addresses by reading the source MAC addresses from
3. Explain that a bridging table maps the MAC addresses on each segment to the
corresponding port on the bridge to which each segment is connected.
5. When you segment a LAN with one or more bridges, remember these points:
a. Bridges reduce collisions on the LAN and filter traffic based on MAC addresses.
Segmenting with Routers
1. Explain that a router operates at layer 3 of the OSI reference model. It interprets the
Network layer protocol and makes forwarding decisions based on the layer 3 address.
3. Explain that routers maintain routing tables that include the Network layer addresses of
different segments.
4. Keep in mind that when you segment a LAN with routers, they will:
a. Decrease collisions by filtering traffic
b. Reduce broadcast and multicast traffic by blocking or selectively filtering
packets
c. Support multiple paths and routes between them
d. Provide increased bandwidth for the newly created segments
Quick Quiz 1
1. A(n) ____ occurs when two stations transmit more than 64 bytes of data frames before
detecting a collision.
2. In ____ communications, devices can send and receive signals, but not at the same time.
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Answer: half-duplex
3. A(n) ____ segments a network by filtering traffic at the Data Link layer.
4. A(n) ____ operates at layer 3 of the OSI reference model. It interprets the Network
layer protocol and makes forwarding decisions based on the layer 3 address.
LAN Switching
1. Explain that although switches are similar to bridges in several ways, using a switch on
a LAN has a different effect on the way network traffic is propagated.
Segmentation with Switches
1. Explain that switches are often called multiport bridges. A switch typically connects
2. Mention that switches microsegment the network by connecting each port to an
3. Explain that switch latency is typically higher than that of a repeater or hub. Faster
4. Describe the benefits provided by switches, including the following:
a. Reduction in network traffic and collisions
b. Increase in available bandwidth per station
c. Increase in the effective distance of a LAN by dividing it into multiple collision
domains
d. Increased security, because unicast traffic is sent directly to its destination
Teaching
Tip
Because switches microsegment most traffic, they utilize available bandwidth
more efficiently. When one host is communicating directly with another host, the
connection’s full bandwidth is available to them.
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Switch Operations
1. Explain that a switch learns the hardware address of devices to which it is attached by
reading the source address of frames as they are transmitted across the switch. The
3. Describe the following types of memory buffering:
4. Explain that some switches can interconnect network interfaces of different speeds
using asymmetric switching. With symmetric switching, switches require all attached
network interface devices to use the same transmit/receive speed.
Switching Methods
1. Mention that all switches base frame-forwarding decisions on a frame’s destination
2. One additional forwarding method, adaptive cut-through forwarding, is a combination
of the cut-through and store-and-forward methods.
Cut-Through Forwarding
1. Explain that switches that use cut-through forwarding start sending a frame immediately
2. The main benefit of cut-through forwarding is a reduction in latency. The drawback is
the potential for errors in the frame that the switch would be unable to detect because
the switch only reads a small portion of the frame into its buffer.
Store-and-Forward Forwarding
1. Explain that store-and-forward switches read the entire frame, no matter how large, into
their buffers before forwarding. Use Figure 12-6 to illustrate your explanation.
2. Mention that because the switch reads the entire frame, it will not forward frames with
errors. The store-and-forward method has the highest latency.
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Teaching
Tip
Standard bridges typically use the store-and-forward technique.
Fragment-Free Forwarding
1. Explain that fragment-free forwarding represents an effort to provide more error-
2. Mention that a fragment-free switch reads the first 64 bytes of an Ethernet frame and
3. Explain that for the most part, the adaptive cut-through switch will act as a cut-through
switch to provide the lowest latency. However, if a certain level of errors is detected,
the switch will change forwarding techniques and act more as a store-and-forward
switch.
Switch User Interface
2. Explain that you can connect to a Cisco switch in the same way you connect to a Cisco
router. The Cisco switch has a console port to which you can connect your laptop or PC.
Once you power on the switch you will be in the command-line interface; you can
configure anything from the command line.
Modes and Passwords
1. Mention that you cannot actually configure a switch until you get to enable mode. To
enter enable mode, type enable at the command-line prompt and then press Enter.
Setting the Host Name
2. Explain that to configure this name, you would type: Switch(config)#hostname
name. Once the host name is set, the prompt will change to reflect the name of the
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IP on the Switch
1. Mention that by default, Cisco switches are not configured with IP addresses. Generally
speaking, a switch does not require an IP address, because switches operate mainly on
2. Explain that you may want to configure an IP address for your switch so that you can
manage it over the network. Also, you may need to configure an IP address for your
switch if you want to implement VLANs on your network.
Configuring Switch Ports
1. Mention that to enter interface configuration mode for the first port of a switch named
Rm410HL, you would use the following commands:
2. To view the configuration of a port, use the show command.
3. Explain that you would use the following command to set the duplex mode:
a. Rm410HL#configure terminal
Securing Switch Ports
1. Explain that you can choose from several degrees of security on a switch:
a. First, you can configure a permanent MAC address for a specific port on your
5. To clear the settings to include erasing the static MAC addresses, use the clear
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Quick Quiz 2
1. By connecting each port to an individual workstation, switches ____ the network.
2. The MAC-to-switch-port mapping is stored in the switch’s ____.
3. Switches that require all attached network interface devices to use the same
transmit/receive speed are using ____ switching.
4. ____ forwarding represents an effort to provide more error-reducing benefits than cut-
through switching, while keeping latency lower than does store-and-forward switching.
Class Discussion Topics
1. What are the main differences between bridges and switches?
2. What is the main purpose of a collision domain?
Additional Projects
1. A switch is a device that connects devices on a LAN and segments collision domains by
port. Ask your students to read more about LAN switches and write a report explaining
how they work.
2. Ask your students to read more about Ethernet and write a report explaining the basic
Additional Resources
1. Ethernet
2. Fast Ethernet
3. Network Bridges
4. Network switch
5. Network Devices
Key Terms
➢ 100Base-FX A Fast Ethernet implementation over multimode fiber-optic (MMF)
cabling. The maximum segment length is 412 meters.
switches to store-and-forward switching until the error rate returns to an acceptable
level. Also known as error sensing.
➢ alignment error A frame that has both an FCS error and an entire octet missing from
the frame.
➢ asymmetric switching A type of LAN switching that allows for multiple speeds of
➢ bridge protocol data unit (BPDU) Data packets sent between switches that support the
spanning tree protocol.
➢ bridging table A table maintained on a bridge that maps MAC addresses to the bridge
port through which they can be accessed.
➢ broadcast A frame that is addressed to all stations on the broadcast domain. The
simultaneously, a collision will occur, and the stations must detect the collision and
reset themselves.
➢ carrier signal A transmitted electromagnetic pulse or wave on the network wire that
indicates a transmission is in progress.
➢ collision domain The area on a network in which collisions can occur; a section of the
latency, but does not reduce packet errors.
➢ error sensing See adaptive cut-through.
➢ Ethernet See Carrier Sense Multiple Access with Collision Detection (CSMA/CD).
➢ Fast Ethernet Defined in IEEE 802.3u, and includes any of the following 100-Mbps
Ethernet LAN technologies: 100Base-T4, 100Base-TX, 100Base-FX.
➢ gateway A combination of software and hardware that translates between different
protocol suites.
➢ giant See long frame.
➢ half-duplex A connection that allows communication in two directions, but not
simultaneously; the circuit can be used for sending or receiving bits in only one
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➢ late collision A situation that occurs when two stations transmit more than 64 bytes of
their frames before detecting a collision.
➢ latency The lag or delay that a device or part of the network media causes; for example,
fiber-optic cable delays a transmitted signal 1 bit time every 10 meters.
➢ multiport bridge Another name for a switch.
➢ network access method The process by which network interface cards and devices
communicate data on a network; an example is CSMA/CD. Also known as media
access method.
➢ NIC error An error that indicates a NIC is unable to transmit or receive a packet.
➢ runt See short frame.
➢ shared memory buffering Dynamic memory buffer that is shared by all switch ports
and allocated according to the needs of the ports; ports that have more activity and
larger frames to process are allowed to use more memory buffer space.
➢ short frame frame that is smaller than the 64-byte minimum frame transmission size
greatest error reduction, but the highest latency. See cut-through and adaptive cut-
through.
➢ switch A device that connects devices on a LAN and segments collision domains by
port.
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1492.
➢ transmission time The time it takes for a transmission to go from the source host to the