Specifications

Why Would I Use VLANs in My Network?
March 15, 2011 Page 2 of 20
Theprimarybenefitof802.1QVLANtechnologyisthatitallowsyoutolocali zeandsegregrate
traffic,improvingyouradministrativeefficiency,andenhancingyournetworksecurityand
performance.
Figure 1showsasimpleexampleofusingport‐basedVLANstoachievethesebenefits.Inthis
example,twobuildingshousetheSalesand
Financedepartmentsofasinglecompany,andeach
buildinghasitsowninternalnetwork.Theendstationsineachbuildingconnecttoaswitchonthe
bottomfloor.Thetwoswitchesareconnectedtooneanotherwithahighspeedlink.
Figure 1 VLAN Business Scenario
WithoutanyVLANsconfigured,theentirenetworkinthe
exampleinFigure 1wouldbea
broadcastdomain,andtheswitcheswouldfollowtheIEEE802.1Dbridgingspecificationtosend
databetweenstations.AbroadcastormulticasttransmissionfromaSalesworkstationinBuilding
OnewouldpropagatetoalltheswitchportsonSwitchA,crossthehighspeedlink
toSwitchB,
andthenbepropagatedoutallswitchportsonSwitchB.Theswi tches treat eachportasbeing
equivalenttoanyotherport,andhavenounderstandingofthedepartmentalmembershipsof
eachworkstation.
OnceSalesandFinanceareplacedontwoseparateVLANs,eachswitchunderstandsthat
certain
individualportsorframesaremembersof separateworkgroups.Inthisenvironment,abroadcast
ormulticastdatatransmi ssionfromoneoftheSalesstationsinBuildingOnewouldreachSwitch
A,besenttotheportsconnectedtootherlocalmembersoftheSalesVLAN,crossthehighspeed
link
toSwitchB,andthenbesenttoanyotherportsandworkstationsonSwitchBthatare
membersoftheSalesVLAN.SeparateVLANsalsoprovidesunicastseparationbetweenSalesand
Finance.FinancecannotpingSalesunlessthereisaroutedVLANconfiguredforbothFinanceand
Sales.
Anotherbenefit
toVLANuseintheprecedingexamplewouldbeyourabilitytoleverageexisting
investmentsintimeandequipmentduringcompanyreorganization.If,forinstance,theFinance
userschangelocationbutremaininthesameVLANconnectedtothe sameswitchport,their
networkaddressesdonotchange,andswitch
androuterconfigurationisleftintact.
1
SS
FF
A
6
SSS
FF
B
7
8
9
10
Building One Building Two
SmartSwitch SmartSwitch
trunk
S
Member of Sales Network
Member of Finance Network
S
F