1-2Figure 1-1 A VLAN implementationAdvantages of VLANsCompared with traditional Ethernet technology, VLAN technology delivers the following benefits:z Confining broadcast traffic within individual VLANs. This saves bandwidth and improves networkperformance.z Improving LAN security. By assigning user groups to different VLANs, you can isolate them atLayer 2. To enable communication between VLANs, routers or Layer 3 switches are required.z Flexible virtual workgroup creation. As users from the same workgroup can be assigned to thesame VLAN regardless of their physical locations, network construction and maintenance is mucheasier and more flexible.VLAN FundamentalsVLAN tagTo enable a Layer-2 switch to identify frames of different VLANs, a VLAN tag field is inserted into thedata link layer encapsulation.The format of VLAN-tagged frames is defined in IEEE 802.1Q issued by IEEE in 1999.In the header of a traditional Ethernet data frame, the field after the destination MAC address and thesource MAC address (DA&SA) is the Type field indicating the upper layer protocol type, as shown inFigure 1-2.Figure 1-2 Encapsulation format of traditional Ethernet framesIEEE 802.1Q inserts a four-byte VLAN tag after the DA&SA field, as shown in Figure 1-3.
