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Configuring VLANs
Introduction to VLAN
VLAN overview
Ethernet is a network technology based on the Carrier Sense Multiple Access/Collision Detect
(CSMA/CD) mechanism. As the medium is shared in an Ethernet, network performance may degrade as
the number of hosts on the network is increasing. If the number of the hosts in the network reaches a
certain level, problems caused by collisions, broadcasts, and so on emerge, which may cause the
network to malfunction. In addition to the function that suppresses collisions (which can also be achieved
by interconnecting LANs), virtual LAN (VLAN) can isolate broadcast packets as well. VLAN divides a
LAN into multiple logical LANs with each being a broadcast domain. Hosts in the same VLAN can
communicate with each other like in a LAN. However, hosts from different VLANs cannot communicate
directly. In this way, broadcast packets are confined to a single VLAN, as illustrated in the following
figure.
Figure 1
A VLAN diagram
A VLAN can span across physical spaces. The hosts that reside in different network segments may
belong to the same VLAN, users in a VLAN can be connected to the same switch, or span across multiple
switches or routers.
VLAN technology has the following advantages:
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Broadcast traffic is confined to each VLAN, reducing bandwidth utilization and improving network
performance.
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LAN security is improved. Packets in different VLANs are isolated at Layer 2. That is, users in a
VLAN cannot communicate with users in other VLANs directly, unless Layer 3 network devices such
as routers are used.
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A more flexible way to establish virtual workgroups. With VLAN technology, a virtual workgroup
can be created spanning physical network segments. That is, users from the same workgroup do not
have to be within the same physical area, making network construction and maintenance much
easier and more flexible.