Specifications

Understanding How VLANs Operate
March 15, 2011 Page 5 of 20
Ingress Precedence
VLANassignmentforreceived(ingress)framesisdeterminedbythefollowingprecedence:
1. 802.1QVLANtag(taggedframesonly).
2. PolicyorTrafficClassification(whichmayoverwritethe802.1QVLANtag).Formore
information,referto“ConfiguringProtocol‐BasedVLANClassification”onpage 16.
3. PortVLANID(PVID).
Forwarding Decisions
VLANforwardingdecisionsfortransmittingframesisdeterminedbywhetherornotthetraffic
beingclassifiedisorisnotintheVLAN’sforwardingdatabaseasfollows:
• Unlearnedtraffic:Whenaframe’sdestinationMACaddressisnotintheVLAN’sforwarding
database(FDB),itwillbeforwardedoutofevery
portontheVLAN’segresslistwiththe
frameformatthatisspecified.Referto”Broadcasts,Multicasts,andUnlearnedUnicasts“
belowforanexample.
• Learnedtraffic:Whenaframe’sdestinationMACaddressisintheVLAN’sforwarding
database,itwillbeforwardedoutofthelearnedportwiththeframe
formatthatisspecified.
Referto“LearnedUnicasts”belowforanexample.
Broadcasts, Multicasts, and Unlearned Unicasts
Ifaframewithabroadcast,multicast,orotherunknownaddressisreceivedbyan802.1QVLAN
awareswitch,theswitchcheckstheVLANclassificationoftheframe.Theswitchthenforwards
theframeoutallportsthatareidentifiedintheForwardingListforthatVLAN.Forexample,if
Port
3,shownintheexampleinFigure 2,receivedtheframe,theframewouldthenbesenttoall
portsthathadVLANCintheirPortVLANList.
Learned Unicasts
WhenaVLANswitchreceivesaframewithaknownMACaddressasitsdestinationaddress,the
actiontakenbytheswitchtodeterminehowtheframeistransmitteddependsontheVLAN,the
VLANassociatedFID,andiftheportidentifiedtosendtheframeisenabledtodoso.
Whenaframeisreceived,itisclassifiedintoaVLAN.Thedestinationaddressislookedupinthe
FIDassociatedwiththeVLAN.Ifamatchisfound,itisforwardedouttheportidentifiedinthe
lookupif,andonlyif,thatportisallowedtotransmitframesfor
thatVLAN.Ifamatchisnot
found,thentheframeisfloodedoutallportsthatareallowedtotransmitframesbelongingtothat
VLAN.
Example of a VLAN Switch in Operation
Theoperationofan802.1QVLANswitchisbestunderstoodfromapointofviewoftheswitch
itself.Toillustratethisconcept,theexamplesthatfollowviewthe switchoperationsfrominside
theswitch.
Figure 2depictstheinside ofaswitchwithsixports,numbered1through6.Theswitch
hasbeen
configuredtoassociateVLANAandBwithFID2,VLANCandDwithFID3,andVLANEwith
FID4.Itshowshowaforwardingdecisionismadebycomparingaframe’sdestinationMACto
theFIDtowhichitisclassified.