138Configuring VLANsThis chapter provides an overview of VLANs and explains how to configure them.OverviewEthernet is a family of shared-media LAN technologies based on the CSMA/CD mechanism. An EthernetLAN is both a collision domain and a broadcast domain. As the medium is shared, collisions andbroadcasts are common in an Ethernet LAN. Typically, bridges and Layer 2 switches can reducecollisions in an Ethernet LAN. To confine broadcasts, a Layer 2 switch must use the Virtual Local AreaNetwork (VLAN) technology.VLANs enable a Layer 2 switch to break a LAN down into smaller broadcast domains, as shownin Figure 38.Figure 38 A VLAN diagramA VLAN is logically divided on an organizational basis rather than on a physical basis. For example, youcan assign all workstations and servers used by a particular workgroup to the same VLAN, regardless oftheir physical locations. Hosts in the same VLAN can directly communicate with one another. You needa router or a Layer 3 switch for hosts in different VLANs to communicate with one another.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 tag between thedestination and source MAC address (DA&SA) field and the Type field.VLAN 2VLAN 5Switch BSwitch ARouter
