3com TR User Manual

C

HAPTER

 7: V

IRTUAL

 LAN

S

Implementing VLANs on your network has three main 
advantages:

■

It eases the change and movement of devices on 
networks.

■

It helps to control broadcast traffic.

■

It provides extra security.

With traditional IP networks, network administrators 
spend much of their time dealing with moves and 
changes. If users move to a different IP subnet, the IP 
addresses of each endstation must be updated manu-
ally. 

With a VLAN setup, if an endstation in VLAN 1 is 
moved to a port in another part of the network, you 
only need to specify that the new port is in VLAN 1, 
and this is something that can be done remotely if 
you have 3Com’s Transcend Enterprise Manager for 
Windows (v6.0 and above).

With traditional networks, congestion can be caused 
by broadcast traffic that is directed to all network 
devices whether they require it or not. VLANs increase 
the efficiency of your network because each VLAN 
can be set up to contain only those devices that need 
to communicate with each other.

Devices within each VLAN can only communicate 
with devices in the same VLAN. If a device in VLAN 1 
needs to communicate with devices in VLAN 2, the 
traffic must cross a router. Routers have filtering and 
security features to block unauthorized access.

Figure 7-1 shows a network configured with three 
VLANs—one for each of the departments that access 
the network. The membership of VLAN 1 is restricted 
to ports 1, 2, 3, 4 and 5 of Switch A; membership of 
VLAN 2 is restricted to ports 4, 5, 6, 7 and 8 of Switch 
B while VLAN 3 spans both Switches containing ports 
6, 7, 8 of Switch A and 1, 2, 3 of Switch B. 

In this simple example, each of these VLANs can be 
seen as a ’broadcast domain’—physical LAN seg-
ments that are not constrained by their physical loca-
tion.

Specific configurations using the Switch are shown 
later in this chapter.