114Configuring VLANsOverviewEthernet is a family of shared-media LAN technologies based on the CSMA/CD mechanism. AnEthernet LAN is both a collision domain and a broadcast domain. Because the medium is shared,collisions and broadcasts are common in an Ethernet LAN. Typically, bridges and Layer 2 switchescan reduce collisions in an Ethernet LAN. To confine broadcasts, a Layer 2 switch must use theVirtual Local Area Network (VLAN) technology.VLANs enable a Layer 2 switch to break a LAN down into smaller broadcast domains, as shownin Figure 36.Figure 36 A VLAN diagramA VLAN is logically divided on an organizational basis rather than on a physical basis. For example,you can assign all workstations and servers used by a particular workgroup to the same VLAN,regardless of their physical locations. Hosts in the same VLAN can directly communicate with oneanother. You need a router or a Layer 3 switch for hosts in different VLANs to communicate with oneanother.All these VLAN features reduce bandwidth waste, improve LAN security, and enable flexible virtualgroup creation.VLAN frame encapsulationTo identify Ethernet frames from different VLANs, IEEE 802.1Q inserts a four-byte VLAN tagbetween the destination and source MAC address (DA&SA) field and the Type field.Figure 37 VLAN tag placement and formatA VLAN tag includes the following fields:• TPID—16-bit tag protocol identifier that indicates whether a frame is VLAN-tagged. By default,the TPID value 0x8100 identifies a VLAN-tagged frame. A device vendor can set the TPID fieldVLAN 2VLAN 5Switch BSwitch ARouter
