Advantages Of Vlans; Vlan Principles - 3Com E4500-24 Cli Configuration Manual

Figure 1-1 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
To enable a network device to identify frames of different VLANs, a VLAN tag field is inserted into the
data link layer encapsulation.
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 1-2 Encapsulation format of traditional Ethernet frames
In Figure 1-2 DA refers to the destination MAC address, SA refers to the source MAC address, and Type
refers to the upper layer protocol type of the packet. IEEE 802.1Q protocol defines that a 4-byte VLAN
Figure 1-2
1-2