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C

HAPTER

 4: VLAN O

VERVIEW

Figure 29   A VLAN implementation

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 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