3Com WX3000 operation manual Advantages of VLANs, How Vlan Works, Vlan tag

of network layer devices, such as routers and Layer 3 switches. Figure 1-1 illustrates a VLAN implementation.

Figure 1-1A VLAN implementation

Router

Switch

Switch

VLAN A	VLANB	VLAN A	VLANB
	VLAN A	VLAN B

Advantages of VLANs

Compared with the traditional Ethernet, VLAN enjoys the following advantages.

z

z

z

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.

How VLAN Works

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 1-2

1-2