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Chapter 6 VLAN
The traditional Ethernet is a data network communication technology basing on CSMA/CD (Carrier
Sense Multiple Access/Collision Detect) via shared communication medium. Through the
traditional Ethernet, the overfull hosts in LAN will result in serious collision, flooding broadcasts,
poor performance or even breakdown of the Internet. Though connecting the LANs through
switches can avoid the serious collision, the flooding broadcasts can not be prevented, which will
occupy plenty of bandwidth resources, causing potential serious security problems.
A Virtual Local Area Network (VLAN) is a network topology configured according to a logical
scheme rather than the physical layout. The VLAN technology is developed for switches to control
broadcast in LANs. By creating VLANs in a physical LAN, you can divide the LAN into multiple
logical LANs, each of which has a broadcast domain of its own. Hosts in the same VLAN
communicate with one another as if they are in a LAN. However, hosts in different VLANs cannot
communicate with one another directly. Therefore, broadcast packets are limited in a VLAN. Hosts
in the same VLAN communicate with one another via Ethernet whereas hosts in different VLANs
communicate with one another through the Internet devices such as Router, the Layer3 switch, etc.
The following figure illustrates a VLAN implementation.
Figure 6-1 VLAN implementation
Compared with the traditional Ethernet, VLAN enjoys the following advantages.
（1） Broadcasts are confined to VLANs. This decreases bandwidth utilization and improves
network performance.
（2） Network security is improved. VLANs cannot communicate with one another directly. That
is, a host in a VLAN cannot access resources in another VLAN directly, unless routers or
Layer 3 switches are used.
（3） Network configuration workload for the host is reduced. VLAN can be used to group
specific hosts. When the physical position of a host changes within the range of the VLAN,
you need not to change its network configuration.
A VLAN can span across multiple switches, or even routers. This enables hosts in a VLAN to be
dispersed in a looser way. That is, hosts in a VLAN can belong to different physical network
segment. This switch supports three ways, namely, 802.1Q VLAN, MAC VLAN and Protocol VLAN,
to classify VLANs. VLAN tags in the packets are necessary for the switch to identify packets of
different VLANs. The switch can analyze the received untagged packets on the port and match the
packets with the MAC VLAN, Protocol VLAN and 802.1Q VLAN in turn. If a packet is matched, the
switch will add a corresponding VLAN tag to it and forward it in the corresponding VLAN.
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