3com WX3000 Manuel D’Utilisation

 

1-2 

of network layer devices, such as routers and Layer 3 switches. 

 illustrates a VLAN 

implementation. 

Figure 1-1 A VLAN implementation 

Switch

Router

Switch

VLAN A

VLANB

VLAN A

VLANB

VLAN A

VLAN B

 

 

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

z 

Broadcasts are confined to VLANs. This decreases bandwidth consumption and improves network 

performance. 

z 

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.  

z 

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