Vlan Overview; Introduction To Vlan - 3Com 4200G 12-Port Configuration Manual

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VLAN Overview

This chapter covers these topics:
VLAN Overview
Port-Based VLAN
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. A switch builds a table of MAC addresses mapped to associated ports with that address
and only sends a known MAC's traffic to one port. 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.
Related to the point above, someone on a network can monitor broadcast packets and unicast
packets and learn of other activities on the network. Then they can attempt to access other
resources on the network, whether or not they are authorized to do this.
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 multiple 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
of network layer devices, such as routers and Layer 3 switches.
implementation.
1-1
Figure 1-1 illustrates a VLAN