3Com Switch 4210 9-Port, Configuration Manual

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74 C
HAPTER
4: VLAN O
VERVIEW
Figure 29 A VLAN implementation
Advantages of VLANs Compared with the traditional Ethernet, VLAN enjoys the following advantages.
■
Broadcasts are confined to VLANs. This decreases bandwidth consumption and
improves network performance.
■
Network security is improved. Because each VLAN forms a broadcast domain,
hosts in different VLANs cannot communicate with each other directly unless
routers or Layer 3 switches are used.
■
A more flexible way to establish virtual workgroups. VLAN can be used to
create a virtual workgroup spanning physical network segments. When the
physical position of a host changes within the range of the virtual workgroup,
the host can access the network without changing its network configuration.
VLAN Principles VLAN tag
VLAN tags in the packets are necessary for a switch to identify packets of different
VLANs. A switch works at the data link layer of the OSI model (Layer 3 switches
are not discussed in this chapter) and it can identify the data link layer
encapsulation of the packet only, so you need to add the VLAN tag field into the
data link layer encapsulation if necessary.
In 1999, IEEE issues the IEEE 802.1Q protocol to standardize VLAN
implementation, defining the structure of VLAN-tagged packets.
In traditional Ethernet data frames, the type field of the upper layer protocol is
encapsulated after the destination MAC address and source MAC address, as
shown in Figure 30.
Figure 30 Encapsulation format of traditional Ethernet frames
Switch
Router
Switch
VLAN A VLANB VLAN A VLANB
VLAN A VLAN B
Type Data DA&SA