Overview of VLANs
Definition
Virtual Local Area Network (VLAN) technology divides a physical LAN into multiple broadcast domains, each of which is called a VLAN.
Purpose
Ethernet technology implements data communication over shared media based on Carrier Sense Multiple Access/Collision Detection (CSMA/CD). When an Ethernet network has a large number of hosts, both collisions and broadcast storms become a serious problem, affecting network performance and in some cases causing the network to completely break down. Although using switches to connect LANs can prevent collisions, they cannot isolate broadcast packets or improve network quality.
VLAN technology divides a physical LAN into multiple VLANs to isolate broadcast domains. Because each VLAN functions as a separate broadcast domain, hosts can communicate directly with other hosts only if they are in the same VLAN.
Figure 5-1 shows a simple VLAN networking diagram. Two switches are placed in different locations (for example, in different floors of a building). Each switch is connected to two servers that respectively belong to different VLANs, and the four servers belong to two VLANs.
Benefits
- Limits scope of broadcast domains: A broadcast domain is limited in a VLAN. This saves bandwidth and improves network processing capabilities.
- Enhances LAN security: Packets from different VLANs are transmitted separately, preventing hosts in a VLAN from communicating directly with hosts in another VLAN.
- Improves network robustness: A fault in one VLAN does not affect hosts in other VLANs.
- Allows flexible definition of virtual groups: With VLAN technology, hosts in different geographical locations can be grouped together, thereby simplifying network construction and maintenance.