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C H A P T E R26
Chapter Goals
• Understand the relationship of LAN switching to legacy internetworking devices such as bridges and
routers.
• Understand the advantages of VLANs.
• Know the difference between access and trunk links.
• Know the purpose of a trunk protocol.
• Understand Layer 3 switching concepts.
LAN Switching and VLANs
A LAN switch is a device that provides much higher port density at a lower cost than traditional bridges.
For this reason, LAN switches can accommodate network designs featuring fewer users per segment,
thereby increasing the average available bandwidth per user. This chapter provides a summary of general
LAN switch operation and maps LAN switching to the OSI reference model.
The trend toward fewer users per segment is known as microsegmentation. Micro-segmentation allows
the creation of private or dedicated segments—that is, one user per segment. Each user receives instant
access to the full bandwidth and does not have to contend for available bandwidth with other users. As
a result, collisions (a normal phenomenon in shared-medium networks employing hubs) do not occur, as
long as the equipment operates in full-duplex mode. A LAN switch forwards frames based on either the
frame’s Layer 2 address (Layer 2 LAN switch) or, in some cases, the frame’s Layer 3 address (multilayer
LAN switch). A LAN switch is also called a frame switch because it forwards Layer 2 frames, whereas
an ATM switch forwards cells.
Figure 26-1 illustrates a LAN switch providing dedicated bandwidth to devices and illustrates the
relationship of Layer 2 LAN switching to the OSI data link layer.
26-1
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Chapter 26
LAN Switching and VLANs
History
Figure 26-1 A LAN Switch Is a Data Link Layer Device
History
The earliest LAN switches were developed in 1990. They were Layer 2 devices (bridges) dedicated to
solving desktop bandwidth issues. Recent LAN switches evolved to multilayer devices capable of
handling protocol issues involved in high-bandwidth app