1-2Figure 1-1 A VLAN implementationAdvantages of VLANsCompared with the traditional Ethernet, VLAN enjoys the following advantages.z Broadcasts are confined to VLANs. This decreases bandwidth consumption and improves networkperformance.z Network security is improved. Because each VLAN forms a broadcast domain, hosts in differentVLANs 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 virtualworkgroup spanning physical network segments. When the physical position of a host changeswithin the range of the virtual workgroup, the host can access the network without changing itsnetwork configuration.VLAN PrinciplesVLAN tagTo enable a network device to identify frames of different VLANs, a VLAN tag field is inserted into thedata link layer encapsulation.In 1999, IEEE issues the IEEE 802.1Q protocol to standardize VLAN implementation, defining thestructure of VLAN-tagged packets.In traditional Ethernet data frames, the type field of the upper layer protocol is encapsulated after thedestination MAC address and source MAC address, as shown in Figure 1-2Figure 1-2 Encapsulation format of traditional Ethernet framesIn Figure 1-2 DA refers to the destination MAC address, SA refers to the source MAC address, and Typerefers to the upper layer protocol type of the packet. IEEE 802.1Q protocol defines that a 4-byte VLAN
