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Chapter 4: Ethernet Technology
Core Idea
This chapter is all about the functionality and standards of Ethernet; Ethernet is the most
popular LAN protocol suite in use today.
Instructor Focus
The focus of this topic should be on logical ways for two devices to communicate. The Ethernet
standards include a variety of distance, cabling, and capacity specifications, but they all center
Network+ Exam Objective for this Chapter/Lesson
x 1.3 Explain the concepts and characteristics of routing and switching
Terminology
Make an effort to define the following terms for the designated areas. After defining these,
calling on the students for repeat definitions as you progress through the slide deck will help
them retain the terminology. These terms all fall into one category in this case, Ethernet
functionality.
x Ethernet
x Collision
x Carrier sense multiple access collision detect (CSMA/CD)
x Full duplex
x Half duplex
Focus Questions
x What are the characteristics of Ethernet networks, in terms of media access, collisions
domains, broadcast domains, and distance/speed limitations of various Ethernet standards?
x What functions are performed by Ethernet switch features, such as VLANs, trunks, Spanning
Tree Protocol, link aggregation, Power over Ethernet, port monitoring, user authentication,
and first-hop redundancy?
Icebreaker Discussion
Choose one or both of the following:
1. Why is it necessary to have a common language for communication across devices?
This speaks to the need for Ethernet and why it became so popular; without it you would
have to define the communication structure between any two devices that connect.
2. What are the characteristics of a network cable designated as 100BASET?
It is important for students to identify the characteristics of the Ethernet standard
designations. They will need to understand, recognize, and memorize these for the exam.
Icebreaker Activity
1. On a bus topology, explain the possibility of collisions in transmission. Diagram the topology
to aid in the explanation.
For this activity, you will build the need for collision detection and recovery. This also
Main Activities
1. CSMA/CD
This activity focuses on the Ethernet ability to detect and resolve collisions on the same
segment. Start with a diagram of an Ethernettopology with several devices attached. (This can
be the same topology used in the warm-up activity.) Note that in modern networks, a switch
would be used, but can be represented by the single line to which all the devices are attached.
Because nothing is on the line at the moment, both of the machines believe they are free to
send information.
When both devices transmit at the same time, it destroys both messages on the line. Ethernet
has a means to detect collisions. When it does, it imposes a backoff timer on the devices. With
this, they each choose a random amount of time to wait before checking the line to retransmit.
This feature is called carrier sense multiple access collision detection (CSMA/CD), which breaks
down as follows:
Carrier sense: The ability to listen to the line to determine whether it is clear to send.
Multiple access: There is no deterministic means of transmission, so any device can
send as long as the line is clear.
2. Spanning Tree Protocol
Because Layer 2 data frames do not have a Time-to-Live (TTL) field, it gives rise to the possibility
of broadcast storms on the network. These happen when Layer 2 data frames repeatedly hit
multiple broadcast devices that chain into each other, leading to address table duplication and
Data Frame Duplication:
(Delete this figure above and replace with the one below)
(Note: newly modified Figure to replace the old one)
The following steps compromise the MAC address tables:
2) It is received by both Switch 1(SW1) and Switch 2 (SW2). Both add the MAC address
of PC1 to their MAC Address tables.
4) PC1 and SW2 receive the packet.
6) SW2 rebroadcasts the original data frame from PC1.
8) SW1 now adds the MAC address of PC1 to its MAC Address table. At this point,
both PC1 and PC2 are redundantly sending duplicate messages that are consuming
the bandwidth of the network.
Broadcast Storms:
Using the same configuration, we can demonstrate a broadcast storm on the network. These
will cycle data frames (and packets) and bog down performance for both the network devices
Corrected Figure
The following steps cause a broadcast storm:
1) PC1 sends a broadcast data frameto address FFFF.FFFF.FFFF).
3) SW2 rebroadcasts the message to Segment 2, sending it to PC2 and SW1.
5) SW1 sends the message from SW2 on Segment 2 to Segment 1, sending it back to
PC1 and to SW2.
7) This pattern repeats, sending endless copies to PC1 and PC2 as well as circulating
the message forever on the network.
Spanning Tree Protocol enabled:
The Spanning Tree Protocol can prevent duplicate traffic by ensuring that a single best path is in
use and duplicate paths are unused. This happens through the designation of a root bridge and
nonroot bridges. The root bridge is the one with the lowest bridge ID (BID). Other bridges are
designated as nonroot bridges.
The ports on the bridges are classified as follows:
x Root port: This is the port on a nonroot bridge that is closes to the root bridge (in terms
of network cost).
Quiz Questions
You can ask the following questions either in class for a group discussion or for individual practice. If
you ask them for individual practice, review in class to make sure everyone properly understands
the content.
1. What characteristics are defined by 10BASE-T, 1000BASE-TX, and 10GBASE-SW?
10BASE-T means 10 Mbps, baseband, and UTP wire with a distance limitation of 100 meters.
1000BASE-TX means 1 Gbps, baseband, and UTP with a distance limitation of 100 meters.
2. What is the benefit of VLAN construction?
3. How does Power over Ethernet function and why is it used?
Power over Ethernet (PoE) allows for the transmission of voltage over an Ethernet line. It tests
4. How are Ethernet switches more efficient than the use of bus topology segments?
5. What is a broadcast storm and why is it detrimental?
A broadcast storm occurs when a message sent for broadcast crosses two bridges between the
same two segments of a network. The bridges will repeatedly send the message on one segment
Chapter 4: Network Simulator Activities
Switching Terminology
Connect to Switch Console Port using PuTTY
