Specifications

Understanding How VLANs Operate
March 15, 2011 Page 4 of 20
Learning Modes and Filtering Databases
AddressinginformationtheswitchlearnsaboutaVLANisstoredinthefilteringdatabase
assignedtothatVLAN.Thisdatabasecontainssourceaddresses,theirsourceports,andVLAN
IDs,andisreferredtowhenaswitchmakesadecisionastowheretoforwardaVLANtagged
frame.Eachfilteringdatabase
isassignedaFilteringDatabaseID(FID).
AswitchlearnsandusesVLANaddressinginformationbythefollowingmodes:
• IndependentVirtualLocalAreaNetwork(VLAN)Learning(IVL):EachVLANusesitsown
filteringdatabase.Transparentsourceaddresslearningperformedasaresultofincoming
VLANtrafficisnotmade
availabletoanyotherVLANforforwardingpurposes.Thissetting
isusefulforhandlingdevices(suchasservers)withNICsthatshareacommonMACaddress.
OneFIDisassignedperVLAN.ThisisthedefaultmodeonEnterasysswitches.
• SharedVirtualLocal AreaNetwork(VLAN)Learning(SVL):Twoor
moreVLANsare
groupedtosharecommonsourceaddressinformation.Thissettingisusefulforconfiguring
morecomplexVLANtrafficpatterns,withoutforcingtheswitchtofloodtheunicasttrafficin
eachdirection.ThisallowsVLANstoshareaddressinginformation.Itenablesportsor
switchesindifferentVLANstocommunicatewith
eachother(whentheirindividualportsare
configuredtoallowthistooccur).OneFIDisusedbytwoormoreVLANs.
VLAN Assignment and Forwarding
Receiving Frames from VLAN Ports
Bydefault,Enterasysswitchesrunin802.1Qoperationalmode,whichmeansthateveryframe
receivedbytheswitchmustbelongto,orbeassignedto,aVLAN.Thetypeofframeunder
considerationandthefiltersettingoftheswitchdetermineshowitforwardsVLANframes.This
involvesprocessingtrafficas
itenters(ingresses)andexits(egresses)theVLANswitchportsas
describedbelow.
Untagged Frames
When,forexample,theswitchreceivesaframefromPort1anddeterminestheframedoesnot
currentlyhaveaVLANtag,butrecognizesthatPort1isamemberofVLANA,itwillclassifythe
frametoVLANA.Inthisfashion,alluntaggedframesenteringaVLANswitch
assume
membershipinaVLAN.
Theswitchwillnowdecidewhattodowiththeframe,asdescribedin“ForwardingDecisions”on
page5.
Tagged Frames
When,forexample,theswitchreceivesataggedframefromPort4anddeterminestheframeis
taggedforVLANC,itwillclassifyittothatVLANregardlessofitsportVLANID(PVID).This
framemayhavealreadybeenthroughaVLANawareswitch,ororiginatedfromastation
capable
ofspecifyingaVLANmembership.Ifaswitchreceivesaframecontaining atag,theswitchwill
classifytheframeinregardtoitstagratherthanthePVIDforitsport,followingtheingress
precedenceruleslistedbelow.
Note: A VLAN ID is always assigned to a port. By default, it is the default VLAN (VLAN ID = 1).