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Configuring VLANs
Overview
Ethernet is a network technology based on the CSMA/CD mechanism. Because the medium is
shared, collisions and excessive broadcasts are common on Ethernet networks. To address the
issue, virtual LAN (VLAN) was introduced to break a LAN down into separate VLANs. VLANs are
isolated from each other at Layer 2. A VLAN is a bridging domain, and contains all broadcast traffic
within it.
Figure 32 A VLAN diagram
A VLAN is logically divided on an organizational basis rather than on a physical basis. For example,
using VLAN, all workstations and servers that a particular workgroup uses can be assigned to the
same VLAN, regardless of their physical locations.
VLAN technology delivers the following benefits:
1. Confining broadcast traffic within individual VLANs. This reduces bandwidth waste and
improves network performance.
2. Improving LAN security. By assigning user groups to different VLANs, you can isolate them at
Layer 2. To enable communication between VLANs, routers or Layer 3 switches are required.
3. Creating flexible virtual workgroups. Because users from the same workgroup can be assigned
to the same VLAN regardless of their physical locations, network construction and maintenance
are much easier and more flexible.
VLAN fundamentals
To enable a network device to identify frames of different VLANs, a VLAN tag field is inserted into the
data link layer encapsulation.
The format of VLAN-tagged frames is defined in IEEE 802.1Q issued by the Institute of Electrical and
Electronics Engineers (IEEE) in 1999.
The Ethernet II encapsulation format is used here. Besides the Ethernet II encapsulation format,
Ethernet also supports other encapsulation formats, including 802.2 LLC, 802.2 SNAP, and 802.3
raw. The VLAN tag fields are added to frames encapsulated in these formats for VLAN identification.
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