Vlan Overview - 3Com 7757 Configuration Manual

VLAN O
VERVIEW
10

VLAN Overview

Introduction to VLAN The traditional Ethernet is a flat network, where all hosts are in the same
broadcast domain and connected with each other through hubs or switches. The
hub is a physical layer device without the switching function, so it forwards the
received packet to all ports. The switch is a link layer device which can forward the
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. In this case, a host in the
network receives a lot of packets whose destination is not the host itself. Thus,
plenty of bandwidth resources are wasted, causing potential serious security
problems.
The traditional way to isolate broadcast domains is to use routers. However,
routers are expensive and provide few ports, so they cannot subnet the network
particularly.
The virtual local area network (VLAN) technology is developed for switches to
control broadcast in LANs.
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 with each other as if they are in a LAN. However, hosts in different
VLANs cannot communicate with each other directly. Figure 25 illustrates a VLAN
implementation.