3Com Switch 4800G 24-Port Configuration Manual page 84

84 C 9: VLAN C
HAPTER ONFIGURATION
VLAN Fundamental
Figure 25 A VLAN diagram
VLAN 2
Router
VLAN 5
A VLAN is not restricted by physical factors, that is to say, 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:
1 Broadcast traffic is confined to each VLAN, reducing bandwidth utilization and
improving network performance.
2 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
routers or Layer 3 switches are used.
3 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.
To enable packets being distinguished by the VLANs they belong to, The VLAN tag
fields used to identify VLANs are added to packets. As common switches operate
on the data link layer of the OSI model, they only process data link layer
encapsulation information and the VLAN tag thus needs to be inserted to the data
link layer encapsulation.
The format of the packets carrying the VLAN tag fields is defined in IEEE 802.1Q,
which is issued by IEEE in 1999.
In the header of a traditional Ethernet data frame, the field following the
destination MAC address and the source MAC address is the Type field, which
indicates the upper layer protocol type. Figure 26 illustrates the format of a
traditional Ethernet frame, where DA stands for destination MAC address, SA
stands for source MAC address, and Type stands for the upper layer protocol type
of the frame.
Figure 26 The format of a traditional Ethernet frame
DA&SA
Switch A Switch B
Type Data