Vlan Overview; Introduction To Vlan - 3Com WX3000 Series Operation Manual

1

VLAN Overview

The term switch used throughout this chapter refers to a switching device in a generic sense or the
switching engine of a unified switch in the WX3000 series.
The sample output information in this manual was created on the WX3024. The output information
on your device may vary.
VLAN Overview

Introduction to VLAN

The traditional Ethernet is a broadcast network, where all hosts are in the same broadcast domain and
connected with each other through hubs or switches. Hubs and switches, which are the basic network
connection devices, have limited forwarding functions.
A hub is a physical layer device without the switching function, so it forwards the received packet to
all ports except the inbound port of the packet.
A switch is a link layer device which can forward a packet according to the MAC address of the
packet. However, when the switch receives a broadcast packet or an unknown unicast packet
whose MAC address is not included in the MAC address table of the switch, it will forward the
packet to all the ports except the inbound port of the packet.
The above scenarios could result in the following network problems.
Large quantity of broadcast packets or unknown unicast packets may exist in a network, wasting
network resources.
A host in the network receives a lot of packets whose destination is not the host itself, causing
potential serious security problems.
Isolating broadcast domains is the solution for the above problems. The traditional way is to use routers,
which forward packets according to the destination IP address and does not forward broadcast packets
in the link layer. However, routers are expensive and provide few ports, so they cannot split the network
efficiently. Therefore, using routers to isolate broadcast domains has many limitations.
The virtual local area network (VLAN) technology is developed for switches to control broadcasts in
LANs.
A VLAN can span across physical spaces. This enables hosts in a VLAN to be located in different
physical locations.
By creating VLANs in a physical LAN, you can divide the LAN into multiple logical LANs, each of which
has a broadcast domain of its own. Hosts in the same VLAN communicate in the traditional Ethernet
way. However, hosts in different VLANs cannot communicate with each other directly but need the help
1-1