3Com 2928 User Guide - Page 163

1 VLAN Configuration Overview Introduction to VLAN Ethernet is a network technology based on the Carrier Sense Multiple Access/Collision Detect (CSMA/CD) mechanism. As the medium is shared, collisions and excessive broadcasts are common on an Ethernet. To address the issue, virtual LAN (VLAN) was introduced. The idea is to break a LAN down into separate VLANs, that is, Layer 2 broadcast domains whereby frames are switched between ports assigned to the same VLAN. VLANs are isolated from each other at Layer 2. A VLAN is a bridging domain, and all broadcast traffic is contained within it, as shown in Figure 1-1. Figure 1-1 A VLAN diagram VLANs are logically divided on an organizational basis rather than on a physical basis. For example, all workstations and servers used by a particular workgroup can be connected to the same LAN, regardless of their physical locations. VLAN technology delivers the following benefits: z Confining broadcast traffic within individual VLANs. This reduces bandwidth waste and improves network performance. z Improving LAN security. By assigning user groups to different VLANs, you can isolate them at Layer 2. For hosts in different VLANs to communicate, routers or Layer 3 switches are required. z Flexible virtual workgroup creation. As users from the same workgroup can be assigned to the same VLAN regardless of their physical locations, network construction and maintenance is much easier and more flexible. How VLAN Works To enable a network device to identify frames of different VLANs, a VLAN tag field is inserted into the data link layer encapsulation. 1-1