3Com Switch 4210 9-Port, Configuration Manual

Page: 75 / 567

4
VLAN O
VERVIEW
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 of network layer
devices, such as routers and Layer 3 switches. Figure 29 illustrates a VLAN
implementation.