Operation Manual – VLANH3C S7500 Series Ethernet Switches Chapter 1 VLAN Overview1-2SwitchRouterSwitchVLAN A VLANB VLAN A VLANBVLAN A VLAN BFigure 1-1 A VLAN implementationA VLAN can span across multiple switches, or even routers. This enables hosts in aVLAN to be dispersed in a looser way. That is, hosts in a VLAN can belong to differentphysical network segment.Compared with the traditional Ethernet, VLAN enjoys the following advantages.1) Broadcasts are confined to VLANs. This decreases bandwidth utilization andimproves network performance.2) Network security is improved. VLANs cannot communicate with each otherdirectly. That is, a host in a VLAN cannot access resources in another VLANdirectly, unless routers or Layer 3 switches are used.3) Network configuration workload for the host is reduced. VLAN can be used togroup specific hosts. When the physical position of a host changes within therange of the VLAN, you need not change its network configuration.1.1.2 VLAN PrinciplesVLAN tags in the packets are necessary for the switch to identify packets of differentVLANs. The switch works at Layer 2 (Layer 3 switches are not discussed in this chapter)and it can identify the data link layer encapsulation of the packet only, so you can addthe VLAN tag field into only 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 isencapsulated after the destination MAC address and source MAC address, as shownin Figure 1-2
