Content Overview Local-area network
(LAN) design has developed and changed over time. Network
designers until very recently used hubs and bridges to build
networks. Now switches and routers are the key components in
LAN design, and the capabilities and performance of these
devices are continually improving. This module returns to some
of the roots of modern Ethernet LANs with a discussion of the
evolution of Ethernet/802.3, the most commonly deployed LAN
architecture. A look at the historical context of LAN
development and various networking devices that can be utilized
at Layer 1, Layer 2, and Layer 3 of the OSI model will help
provide a solid understanding of the reasons why network
devices have evolved as they have. Until recently, most
Ethernet networks were built using repeaters. When the
performance of these networks began to suffer because too many
devices shared the same segment, network engineers added
bridges to create multiple collision domains. As networks grew
in size and complexity, the bridge evolved into the modern
switch, allowing microsegmentation of the network. Today’s
networks typically are built using switches and routers, often
with the routing and switching function in the same device.
Many modern switches are capable of performing varied and
complex tasks in the network. This module will provide an
introduction to network segmentation and will describe the
basics of switch operation. Switches and bridges perform much
of the heavy work in a LAN, making nearly instantaneous
decisions when frames are received. This module describes in
detail how frames are transmitted by switches, how frames are
filtered, and how switches learn the physical addresses of all
network nodes. As an introduction to the use of bridges and
switches in LAN design, the principles of LAN segmentation and
collision domains are also covered. Switches are Layer 2
devices that are used to increase available bandwidth and
reduce network congestion. A switch can segment a LAN into
microsegments, which are segments with only a single host.
Microsegmentation creates multiple collision-free domains from
one larger domain. As a Layer 2 device, the LAN switch
increases the number of collision domains, but all hosts
connected to the switch are still part of the same broadcast
domain. Students completing this module should be able to:
- Describe the history and function of shared,
half-duplex Ethernet
- Define collision as it relates to
Ethernet networks
- Define microsegmentation
- Define CSMA/CD
- Describe some of the key elements
affecting network performance
- Describe the function of
repeaters
- Define network latency
- Define
transmission time
- Describe the basic function of Fast
Ethernet
- Define network segmentation using routers,
switches, and bridges
- Describe the basic operations of
a switch
- Define Ethernet switch latency
- Explain the differences between Layer 2 and Layer 3
switching
- Define symmetric and asymmetric
switching
- Define memory buffering
- Compare and
contrast store-and-forward and cut-through switching
- Understand the differences between hubs, bridges, and
switches
- Describe the main functions of switches
- List the major switch frame transmission modes
- Describe the process by which switches learn
addresses
- Identify and define forwarding modes
- Define LAN segmentation
- Define microsegmentation
using switching
- Describe the frame-filtering
process
- Compare and contrast collision and broadcast
domains
- Identify the cables needed to connect switches
to workstations
- Identify the cables needed to connect
switches to switches
Content 4.1
Introduction to Ethernet/802.3 LANs 4.1.1
Ethernet/802.3 LAN development The earliest LAN technologies
commonly used either thick Ethernet or thin Ethernet
infrastructures. It is important to understand some of the
limitations of these infrastructures in order to see where LAN
switching stands today. Adding hubs or concentrators into the
network offered an improvement on thick and thin Ethernet
technology. A hub is a Layer 1 device and is sometimes referred
to as an Ethernet concentrator or a multi-port repeater.
Introducing hubs into the network allowed greater access to the
network for more users. Active hubs also allowed for the
extension of networks to greater distances. A hub does this by
regenerating the data signal. A hub does not make any decisions
when receiving data signals. It simply regenerates and
amplifies the data signals that it receives to all connected
devices. Ethernet is fundamentally a shared technology where
all users on a given LAN segment compete for the same available
bandwidth. This situation is analogous to a number of cars all
trying to access a one-lane road at the same time. Because the
road has only one lane, only one car can access it at a time.
The introduction of hubs into a network resulted in more users
competing for the same bandwidth. Collisions are a by-product
of Ethernet networks. If two or more devices try to transmit at
the same time a collision occurs. This situation is analogous
to two cars merging into a single lane and the resulting
collision. Traffic is backed up until the collision can be
cleared. When the number of collisions in a network is
excessive, sluggish network response times result. This
indicates that the network has become too congested or too many
users are trying to access the network at the same time. Layer
2 devices are more intelligent than Layer 1 devices. Layer 2
devices make forwarding decisions based on Media Access Control
(MAC) addresses contained within the headers of transmitted
data frames. A bridge is a Layer 2 device used to divide, or
segment, a network. A bridge is capable of collecting and
selectively passing data frames between two network segments.
Bridges do this by learning the MAC address of all devices on
each connected segment. Using this information, the bridge
builds a bridging table and forwards or blocks traffic based on
that table. This results in smaller collision domains and
greater network efficiency. Bridges do not restrict broadcast
traffic. However, they do provide greater traffic control
within a network. A switch is also a Layer 2 device and may be
referred to as a multi-port bridge. A switch has the
intelligence to make forwarding decisions based on MAC
addresses contained within transmitted data frames. The switch
learns the MAC addresses of devices connected to each port and
this information is entered into a switching table. Switches
create a virtual circuit between two connected devices that
want to communicate. When the virtual circuit is created, a
dedicated communication path is established between the two
devices. The implementation of a switch on the network provides
microsegmentation. In theory this creates a collision free
environment between the source and destination, which allows
maximum utilization of the available bandwidth. A switch is
also able to facilitate multiple, simultaneous virtual circuit
connections. This is analogous to a highway being divided into
multiple lanes with each car having its own dedicated lane. The
disadvantage of Layer 2 devices is that they forward broadcast
frames to all connected devices on the network. When the number
of broadcasts in a network is excessive, sluggish network
response times result. A router is a Layer 3 device. The router
makes decisions based on groups of network addresses, or
classes, as opposed to individual Layer 2 MAC addresses.
Routers use routing tables to record the Layer 3 addresses of
the networks that are directly connected to the local
interfaces and network paths learned from neighboring routers.
The purpose of a router is to do all of the following:
- Examine incoming packets of Layer 3 data
- Choose
the best path for them through the network
- Switch
them to the proper outgoing port
Routers are not
compelled to forward broadcasts. Therefore, routers reduce the