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TheapplicationpackagesthedataintoaTransportLayersegmentthatistobetransmittedtotheremote.

uniquelyidentifiesthesourceanddestinationnetworkdeviceintheinternetwork.Thepacketmaythen

havetobetransmittedoverseveraldifferentnetworks(same/differentphysicalmedia)beforeitreachesitsdestination.Inanyoneparticularnetwork,theDataLinkLayerisresponsibleforencapsulatingthepacketintoaframeforLayer2forwarding.TheframeisstampedwithaDataLinkheader,whichcontainsDataLinksourceanddestinationaddresses.WhenEthernetisused,theseDataLinkaddressesarecalledmediaaccesscontrol(MAC)addresses.AfteraddingtheDataLinkaddressestotheframe,theDataLinkLayerpassestheframetothephysicallayerfortransmissionoverthephysicalmedium.Thereceivingnetworkdevicemustbeabletorecognizethattheframeisdestinedforitselfandverif thatthe acketisintact.Becausetheentire acketistransmittedoverthephysicalmedium,noiseandothersignaldisturbancescouldcorruptorchangethepacket,renderingitmeaninglesstothehigherlayerapplication.Layer2/DataLinknetworkscanbeclassifiedbroadlyintopointtopointnetworks,circuitbasednetworks,andsharednetworks.Pointtopointnetworkprotocolsdonotusuallyrequireasourceanddestinationaddressessincetheyareestablishedbetweentwonetworkingdevicesonly.TheLayer2framingusuallyconsistsof:

acircuitidentifierinthecaseofcircuitbasednetworksanaddressthatdirectsthepackettotherequireddestination,usuallyonsharedmediaafixedlengthmaximumsize,maximumtransmissionunit(MTU)establishedbetweenthesourceandreceivingcomponent;datafromhigherlayersisbrokenintofixedlengthframes(coveredlater)anerrorcheckthatisinsertedbythesourcecomponentandverifiedbythereceivingcomponenttomaintaindataintegrity

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ThescopeofaDataLinkframeisthelocalnetwork.Forexample,inatypicalscenarioof, .

whileittraversestheLayer2devicesinaparticularIPsubnet.IftheIPpacketneedstobe

routedtoanothersubnetviaanIProuter,theoriginalDataLinkframeisstrippedafteritingressestheIProuter.WhenforwardingtheIPpacketoutfromtheappropriateport,theIProuterconstructsanewDataLinkframewithcorrectheadersandDataLinkaddresses.ThisnewDataLinkheaderisusedastheframetraversestothenextsubnet.Thisprocesscontinuesuntilthedestinationhostisreached.Theapplicationdatasentbetweentwohoststationscantraverseseveralphysicallydifferentnetworks.EachnetworkhasadifferentDataLinkheaderandmayevenusedifferentDataLinkprotocolsthatdependuponthephysicalwire;forexampleEthernet,pointtopointprotocol(PPP),ATM,FrameRelay.Inthisslide,theendhostsontheLayer2networkcommunicatewitheachother,orbywayofLayer2devices,usingthespecificLayer2protocol.ThePCsontheleftsideoftheEthernetnetworkdonotrequireanythingotherthanEthernetL2framingtocommunicatewitheachother.ThePCsontherightsideofthenetworksimilarlyrequireonlyATML2framingtocommunicatewitheachother.TheL2networksareseparatedbyrouters,whichareLayer3OSI

ev ces. e son e erne ne wor canon ycommun ca ew e son enetworkusingLayer3addresses.

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InearliertimesoftheInternet,pointtopointdatalinksallowedhoststocommunicate

witheachotherthroughthetelephonenetwork.OlderprotocolssuchasSLIP(serialline

IP)providedasimplemechanismforframinghigherlayerapplicationsfortransmissionalongseriallines.SLIP,inaccordancewithRFC1055,sentthedatagramacrosstheserial

lineasaseriesofbytes,anditusedspecialcharacterstomarkwhenaseriesofbytes

shouldbegroupedtogetherasadatagram.SLIPwassimpleenoughbutcouldnot

controlthecharacteristicsoftheconnection.

Today,theprotocolofchoiceisPPP,whichprovidesadvantagessuchaslinkcontrolto

negotiatethelinkcharacteristics,networkcontroltotransfermultipleLayer3protocols,

andprovidesauthenticationusedbyremotecomputerstodialintotheirInternet

service.

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PPPisapointtopointdatalinklayerprotocolthatwasinitiallydesignedtotransportIPpackets.Fla :Thefirstfla fieldindicatesthestartofaPPPframe.Italwa shasthevalue01111110binar 0x7Ehexadecimal,or126decimal).ThelastflagfieldindicatestheendofaPPPframe.Italwayshasthevalue 01111110binary(0x7Ehexadecimal,or126decimal).

Address:

InHDLC,theaddressofthedestinationoftheframe.However,inPPPwehaveadirectlinkbetweentwodevices,sothisfieldhasnomeaning.Therefore,itisalwayssetto11111111(0xFFhexadecimal,or255decimal),whichisequivalenttoabroadcast(itmeansallstations).Control:ThisfieldisusedinHDLCforvariouscontrolpurposes,butinPPPitissetto00000011(0003hexadecimal,or3decimal).Data:Zeroormorebytesofpayloadthatcontainseitherdataorcontrolinformation,dependingontheframetype.ForregularPPPdataframes,thenetworklayerdatagramisencapsulatedhere.Forcontrolframes,the controlinformationfieldsareplacedhereinstead.Padding:Insomecases,additionaldummybytesmaybeaddedtopadoutthesizeofthePPPframe.(forexample,

FrameCheckSequence(FCS):Achecksumcomputedovertheframetoprovidebasicprotectionagainsterrorsintransmission.ThischecksumisaCRCcodesimilartotheoneusedforotherlayertwoprotocolerrorprotectionschemes,suchastheoneusedinEthernet.FCScanbeeither16bitsor32bits(defaultis16bits).TheFCSiscalculatedovertheAddress,Control,Protocol,Data,andPaddingfields.Protocol:IdentifiestheprotocolofthedatagramencapsulatedintheDatafieldoftheframe.SeebelowformoreinformationabouttheProtocolfield.Value(inhex) ProtocolName Reference0001 PaddingProtocol0003 ROHCsmallCID [RFC3095]0005 ROHClargeCID [RFC3095]0007to001f reserved(transparencyinefficient)

n erne ro oco vers on0023 OSINetworkLayer0025 XeroxNSIDP0027 DECnetPhase IV0029 Appletalk002b NovellIPX

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Circuitswitchedprotocolsallowthetransferofuserinformationasauniquesetof

packetsidentifiedbyvirtualcircuits.

Intheslide,theswitchontheleftacceptstrafficfromeachhostPCintoavirtualcircuitandswitchestoanothervirtualcircuitwhengoingtotherouter.Thevirtualcircuit

numberisthesamebetweenthehostPCandtheswitch,andbetweentheswitchand

therouter.TrafficfromeachPCisuniquelyidentifiedbyavirtualcircuitateveryhop.

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Applicationpacketsarebrokeninto53bytefixedsizedcellsincludinga5byteheaderalso

ATMcircuitisidentifiedbyaVPI/VCIvalue

EnhancedQoS supportwith5serviceclassesIdealformultipleservicesonthesamelineTheATMheaderconsistsofthefollowingfields:GFC4bitsofgenericflowcontrolthatareusedtoprovidelocalfunctions,suchasidentifyingmultiplestationsthatshareoneATMinterface.TheGFCfieldistypicallynotusedandissettoadefaultvalue.VPI8bitsofvirtualpathidentifierthatisused,inconjunctionwiththeVCI,toidentifythenextdestinationofacellasitpassesthroughaseriesofswitchroutersonitswaytoitsfinaldestination.VCI16bitsofvirtualchannelidentifierthatisused,inconjunctionwiththeVPI,toidentifythenextdestinationofacellasitpassesthroughaseriesofswitchroutersonitswaytoitsfinaldestination.PT3bitsofpayloadtype.Thefirstbitindicateswhetherthecellcontainsuserdataorcontroldata.Ifthecellcontainsuserdata,thesecondbitindicatescongestion,andthethirdbitindicateswhetherthecellisthelastinaseriesofcellsthatrepresentoneAAL5frame.

o ce osspr or y a n ca esw e er ece s ou epre eren a y scar e ifitencounterscongestionasitmovesthroughthenetworkHEC8bitsofheadererrorcontrolthatareachecksumcalculatedonlyontheheader.

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ATMpacketsarefurtherencapsulatedbyATMadaptationlayers(AAL),whichareresponsibleforthesegmentationandreassembl SAR ofATMcellsofhi herla erdatareceivedattheotherend.The ur oseofthisistoada ttheclassofservicefromhigherlayersontoconnectionlessATMcells.TheAALclassificationisrelatedtotheserviceandapplicationrequiredfortransport.Usuallythefollowingadaptationlayersaremappedtothefollowingclassesof

service:AAL1 ConstantBitrateserviceAAL2 VariableBitrateserviceAAL3/4 ConnectionorienteddatausuallyAAL5 Connectionlessdataserviceusually(forexample,IP)ConstantBitRate(CBR)service:AAL1encapsulationsupportsaconnectionorientedservicewhereminimaldatalossisrequired.Examplesofthisserviceinclude64kb/svoice,fixedrateuncompressedvideo,andleasedlinesforprivatedatanetworks.VariableBitRate(VBR)service:AAL2encapsulationsupportsaconnectionorientedserviceinwhichthebitrateis

.video.Therequirementonboundeddelayfordeliveryisnecessaryforthereceivertoreconstructtheoriginaluncompressedvoiceorvideo.Connectionorienteddataservice:Forconnectionorientedfiletransferanddatanetworkapplicationswhereaconnectionissetupbeforedataistransferred,thistypeofservicehasvariablebitrateanddoesnotrequireboundeddelayfordelivery.TwoAALprotocolsweredefinedtosupportthisserviceclassandhavebeenmergedintoonetypecalledAAL3/4.Connectionlessdataservice:Examplesofthisserviceincludedatagramtrafficanddatanetworkapplicationswherenoconnectionissetupbeforedataistransferred.ConnectionlessdataserviceisusedtotransportIP/Ethernet/FrameRelayapplications.HigherlevelServiceDeliveryUnits(SDUs)maybeseveralbytesinlength.However,astheATMpayloadisonly48bytes,theSDUmustbesegmentedintomultiplecellsasitenterstheATMnetwork,thenreassembledwhenitexitstheATMnetwork.ThisfunctionoftheATMadaptationlayerisknownas

. ea ap a on ayercompr ses wosu ayers,oneo w c s e su ayer, eo er e ng econvergencesublayer (CS),whichperformsservicedependentfunctions.

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EachhostPCsendsinformationtotheswitch.Theswitchthentransmitsaframetothe

routerataconstantdatarate(forexample,1.5Mb/s).Thisframenowdividedintomany

fixedtimeslots(24),eachslotcontains64kbits.Eachhostcanoccupyoneormoretimeslotsperframe.

EachhostPCisassignedafixeddatarate.Ifthehostusesonetimeslot,thenits

transmissionis64kbits inthatslot.Becausethepiperateis1.5Mb/s,thehostwillhave

tosupplytheirnext64kbits inthenextframe.

Inthisslide,eachhostPCtransmitsitscharacteristic frame(grey,yellow,purple).The

framesthataretransmittedfromtheswitchcontainseveraltimeslots.Withineachof

theseframesthreeofthetimeslotsareusedbytherespectivehostPCs.

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TimeDivisionMultiplexing(TDM)isadigitaltechnologywhereindividualsignalsareinterleaved.

individualsignalisrepresentedbyonechannelorbymultiplechannels.Thetotaltransmission

bandwidthissplitamongthetimeslots.Thetotalcompositesignalincludesthepayloadbitsforthecomposingchannelsandoverheadbits.TheframestructuresoftheDS1[ANSI95b]andtheEuropeanE1[ITUT98a]signalsareshownabove.TheDS1signalconsistsof24payloadchannelsplusoverhead.Thebasicframeofeachofthesesignalsrepeatsevery125s,thatis,8000timespersecond.With8bitscarriedineachchannel,thisgivesrisetoabasicdatarateof64kb/sforeachchannel.Therequirementforthisdataratestemsfromtheneedtosampletheanalog telephonysignal8000timespersecondandencodingeachsamplein8bits.ADS1framecontains24channels,eachconsistingof8bits,plus1framing/overheadbit,leadingtoatotalof193bits.Becausetheframerepeatsevery125s(or8000timesasecond),thetotalbitrateoftheDS1signalis1.544Mb/s.Similarly,thetotalbitrateoftheE1signalis2.048Mb/s(32channelsof8bits,repeatingevery125s).Widelyusedsignaling examples:DS1/T1,E1,DS3,E3,OC3/STM1,OC12/STM4Othersignaling examples:DS3thatuses28DS1or7DS2or1DS3=45M a uses

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Synchronousopticalnetwork/SynchronousDigitalHierarchy(SONET/SDH)isahigh

bandwidthWANtransporttechnologydevelopedbyBellCommunicationsResearchand

laterstandardizedbyANSIandITU.SONET/SDHissynchronousinnatureandspecifiesframingandmultiplexingatthephysicallayeroftheOSImodel.SONET/SDHwas

originallydesignedtotransportvoicebuthasbeenadaptedtotransportdatabyusing

Layer2framingtechnologiessuchasPPP/HDLCandATM.

SONET/SDHtechnologyistypicallynotimplementedbysmallormediumsized

businesses,becauseofitshighcost.Itismorecommonlyusedbylargeglobal

companies,longdistancecompanieslinkingmetropolitanareasandcountries,orISPs

thatneedtoguaranteefast,reliableaccesstotheInternet.SONET/SDHisparticularly

suitedtoaudio,video,andimagingdatatransmission.Asyoucanimagine,becauseofits

relianceonfiberopticcableanditsredundancyrequirements,SONET/SDHtechnologyis

expensivetoimplement.

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ThebasicSONETsignalisknownassynchronoustransportsignal(STS1)andhasabit

rateof51.84Mb/s.Thisincludesapayloadof50.112Mb/sandanoverheadof1.728

Mb/s.TheSTS1frameis810bytesandistransmittedin125ms,hencethebitrateof51.84Mb/s.

EachSTS1cancarryoneDS3or28DS1frames.Forhigherdatarates,STS1signalis

incrementedatfixedlevelstoSTS3,STS48,andSTS192.Multiplexingcanoccurinone

ormultiplestages.Forexample,anSTS12canbeformedby4STS3s,or12STS1s,or3

STS3sand3STS1s.EachSTS1payloadinaSONETframeisassignedafixedposition

andcanbeextractedwithouthavingtofullydemultiplextheentireframe.Thisisavery

bigadvantageofSONETcomparedtoDS3.

TheSTMframes(STM1,andsoon)usedbySDHareeffectivelyamultipleofSTS3

frames.Theoverheadisidentical,althoughtheterminologyandoverheadusagevaries

somewhatbetweenthestandards.STM1isdesignedtocarryanE3frame.Anumberof

differentstandardshavebeendefinedforthemultiplexingoflowerdatarateswithin

STS1orSTM1frames.

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Theringtopologyis,byfar,themostcommonincurrentserviceprovidernetworks.Itis

commonbecauseitisthemostresilient.Ringsarebasedontwoorfourfibers.

Transmissionisinonedirectionononehalfofthefibers andintheoppositedirectionontheotherhalf.Halfthebandwidthcanbereservedforprotection.Quickrecoveryfroma

fiber cutanywhereontheringcanbeaccomplishedbyswitchingtothesignalbeing

transmittedintheoppositedirection.Ringtopologieshavebeensosuccessfulat

providingreliabletransportthatevenlonghaulcarriersoftenusemultiple,verylarge

circumferenceringsintheirnationwidenetworks.

Add/dropmultiplexers(ADM)areusedatnodesontheringfortrafficoriginationor

termination.Itisnotunusualforringstobeconnectedtootherringsinthatcase,

crossconnectsprovidetheinterconnectionfunction.

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Broadcastnetworkstypicallyusesharedmediatocommunicatetoallthedevicesthat

areattachedtothatsharedmedia.Fordatatobereliablydeliveredfromthesourceto

thedestination,eachofthedevicesonthesharedmediaisidentifiedbyaparticularaddress.Theframethatissourcedfromthesendingdeviceissenttoallthedevices

sharingthemedia(broadcasting).Alldeviceswillreceivetheframebutonlythedevice

whoseaddressappearsintheframeasthedestinationaddresswillinterpretthedata.

Therestofthedeviceswillignorethedata.

Totransmitdatareliably,thesendingdeviceonthesharedmediamustcomposethe

frame,obtaincontrolofthemedia,andtransmittheinformation.Becausethemediais

shared,itispossibleformultiplestationstotransmittheirinformationsimultaneously,

resultinginacollision.Thiscollisioncausesdatacorruption.Dependingontheprotocol

used,analgorithmneedstobefollowedtoensureaminimumnumberofcollisionsand

alsotoensureproperrecoveryfromcollisions.Anexampleofasharedmediaprotocol

thatisverycommonlyusedtodayisEthernet.

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Computersmustcontendfortransmissiontimeonthenetworkmedia.Infact,Ethernet

iscommonlydescribedasacontentionbasedarchitecture.

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EthernetwasoriginallydesignedbytheXeroxCorporation,butthecompanywas

unsuccessfulatlaunchingthetechnologycommercially.LaterXeroxjoinedwithDigital

EquipmentCorporationtocommerciallystandardizeasuiteofnetworkproductsthatwouldusetheEthernettechnology.IntelCorporationlaterjoinedthegroup,knownas

DECIntelXerox(DIX).DIXdevelopedandpublishedthestandardthatwasusedforthe

10Mb/sversionofEthernet.Originally,theonlymediumcapableofhandlingthese

speedswasamultidropthickcoaxialcable.

CarrierSense,MultipleAccess,CollisionDetection(CSMA/CD)isusedtoarbitratethe

accessdevicesusingthesharedmedia.Thisiscoveredindetaillater.

TheIEEEhadstartedproject802,whichwastoprovidetheindustrywithaframework

forstandardizingofLANtechnology.Becausethetechnologywassodiverse,theIEEE

formedworkinggroupsinsupportofthedifferentLANtechnologies.The802.3working

groupwastaskedwithstandardizingLANsbasedontheEthernettechnology.

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Ethernetsupportstwoframetypes,buttheyhavebeenstandardizedsothatalltypescanbe.

(SA)indicateswhethertheframeisEthernetIIor802.3.Ifthevalueis1536orless,theframeis

treatedas802.3.Ifthevalueisgreaterthan1536,theframeistreatedasEthernetII.EthernetIIwasoriginallydevelopedbyDigital,Intel,andXeroxin1980andiscommonlyknownastheDIXstandard.ItwasadoptedbytheIEEEandwentthroughformalstandardizationtoformthe802.3/802.2frametypes.TheEthernetIIframeisusuallyusedfortransmissionofIPdatagrams.Ethernet802.3wasdevelopedbytheIEEEfromtheoriginalEthernetstandardin1983.IEEEEthernetdefinestwolayers;thelowerMAClayerin802.3andanupperLLC(logicallinkcontrol)layerin802.2.Thesearesublayers oftheOSIdatalinklayer(Layer2).ThetwolayersweredefinedseparatelytoprovideadditionallinkcontrolfeaturesandsothatcommonLLCframescouldbeusedfordifferentmediatypes,suchasEthernet,TokenRingandFDDI.ThisallowsbridgingatLayer2betweenthedifferentmediatypes.Therearethreedifferent802.3formatsthatwereusedforolderprotocolssuchasNovelNetwaresIPXandAppleComputersAppletalk protocolsandOSIprotocols.Today,theseformatsarerarelyused.TheAlcatelLucent7750SRusesthe802.3forthetransmissionofISISroutingupdates;however,itusesEthernetIIforothertrafficsuchasIPandMPLS.

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Theframeconsistsofasetofbitsorganizedintoseveralfields.Thesefieldsincludeaddressfields,a,

theintegrityofthebitsintheframetomakesurethattheframehasarrivedintact.TheoriginalEthernet

standardsdefinedtheminimumframesizeas64bytesandthemaximumas1518bytes.ThesenumbersincludeallbytesfromthedestinationMACaddressfieldtotheframechecksequencefield.Thepreambleandthestartframedelimiterfieldsarenotincludedwhenquotingthesizeofaframe.TheIEEE802.3acstandardreleasedin1998extendedthemaximumallowableframesizeto1522bytestoallowforaVLANtagtobeinsertedintotheEthernetframeformat.GigabitEthernetand10gigabitEthernetportsmaysupportjumboframes,whichcanbe9000bytes.Preamble:Astreamofbitsthatallowsthetransmitterandreceivertosynchronizetheircommunication. The reambleisa56bitlon atternofalternatin onesandzeroes.The reambleisimmediatel followedbytheStartFrameDelimiter.StartFrameDelimiter(SFD):Always10101011andisusedtoindicatethebeginningoftheframeinformation.DestinationMAC(DA):TheMACaddressofthemachinereceivingdata.SourceMAC(SA):TheMACaddressofthemachinetransmittingdata.Length/Type:Thepayloadlengthortypefield,(alsoknownasEthertype).IftheEthernetframeisinthe

802.3format,thisfieldisinterpretedaslength.IftheEthernetframeisintheEthernetIIororiginalDIXformat,thefieldisinterpretedastype,orEthertype.Thenumericvalueinthisfielddetermineswhethertheframeisan802.3frameorEthernetIIframe.Ifthevalueislessthan1536,itisan802.3frame.Ifthevalueis1536orgreaterifitisanEthernetIIframe.(continuedonnextslide...)

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(...continuedfrompreviousslide)

overEthernet.ThisfieldcontainsIPXinformationifyouarerunningIPX/SPX(Novell).Containedwithinthe

payloadsectionofanIEEE802.2framearefourspecificfields:DSAPDestinationServiceAccessPointSSAPSourceServiceAccessPointCTRLControlbitsforEthernetcommunicationNLI NetworkLayerInterfaceAnEthernetframemustbeaminimumof64byteslong.Therefore,ifthedatafieldislessthan46bytesinlength,paddingisincludedtobringtheframelengthto64bytes.FrameCheckSe uence FCS :A artoftheframethatverifiesthattheinformationeachframecontainsisnotdamagedduringtransmission.Ifaframeisdamagedduringtransmission,theFCSontheframewillnotmatchwiththerecipient'scalculatedFCS.TheFCSiscalculatedbythesenderbasedontheentirecontentsoftheframe.TherecipientcalculatesanexpectedFCSvalueontheframethatitreceives.AnyframesthatdonotmatchthecalculatedFCSarediscarded.

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ThisslideshowsanactualsniffertraceofanEthernetpacket.Detailsofthistraceareasfollows: ,

EthernetII,Src:FoundryN_d4:a5:00(00:e0:52:d4:a5:00), Dst:Dell_45:61:23(00:11:43:45:61:23)

Destination:Dell_45:61:23(00:11:43:45:61:23)Source:FoundryN_d4:a5:00(00:e0:52:d4:a5:00)Type:IP(0x0800)InternetProtocol,Src:138.120.53.254(138.120.53.254), Dst:138.120.53.149(138.120.53.149)Version:4Headerlength:20bytesDifferentiatedServicesField:0x00(DSCP0x00:Default;ECN:0x00)TotalLength:289I enti ication:0x0ea 3755Flags:0x00Fragmentoffset:0Timetolive:64Protocol:TCP(0x06)Headerchecksum:0xeaa8[correct]Source:138.120.53.254(138.120.53.254)Destination:138.120.53.149(138.120.53.149)TransmissionControlProtocol,Src Port:23(23),Dst Port:2389(2389),Seq:4,Ack:1,Len:249

Destinationport:2389(2389)Sequencenumber:4(relativesequencenumber)Nextsequencenumber:253(relativesequencenumber)Acknowledgementnumber:1(relativeack number)Headerlength:20bytesFlags:0x0018(PSH,ACK)Windowsize:16384Checksum:0xbc0e[correct]Telnet

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EthernetresidesattheDataLinklayer.Thislayercanbesubdividedfurtherintotwo

sublayers:

LLC logicallinkcontrol802.2MAC mediaaccesscontrol

TheLLCinterfacesbetweenthenetworkinterfacelayerandthehigherL3protocoland

mayprovideadditionalfunctionssuchasflowcontrol.LLCisonlyusedwith802.3

Ethernet.ItisnotusedwithEthernetII.

TheMAClayerisresponsiblefordeterminingthephysicalsourceanddestination

addressesforaparticularframeandforthereliabletransferofdata,synchronizationof

datatransmission,errorcontrol,andflowof

data.

Atthephysicallayer,toobservethephysicallinkcondition,Ethernetusesthelink

integritytest,inwhichEthernettransceiverscontinuallymonitorthedatapathfor

activity.Thephysicallayerstandardsalsodefinetheformatoftheelectricaloroptical

signaling thatisusedtorepresentthebinaryonesandzeroesonthetransmission

media.

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OUI=Organisation UniqueIdentifier

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Inthisslide,anEthernetframeiscomposedbythesourcewiththefollowingsourceand

destinationaddresses:

Src :00:e0:b1:88:0d:c0Dest :Dell_c5:79:87(00:14:22:c5:79:87)

Theframeissenttoahubthatconnectsalldevicesona4nodeLAN.Thehubbeinga

simplereplicator,sendstheframeoutonallitsportsexcepttheportwheretheframe

wasreceived(theportattachedtothesource).

Althoughalldevicesreceivetheframe,onlythedevicewhoseMACaddressmatchesthe

destinationdeviceacceptstheframe.

TheoutputsampleshowstheuseofanEthernetframedestinedforaunicast address.

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Inthisslide,anEthernetframeiscomposedbythesourcewiththefollowingsourceand

destinationaddresses:

Src :00:13:ce:2b:6b:28Dest :ff:ff:ff:ff:ff:ff

Theframeissenttoahubthatconnectsalldevicesona4nodeLAN.Thehubbeinga

simplereplicator,sendstheframeoutonallitsportsexcepttheportwheretheframe

wasreceived(theportattachedtothesource).

Alldevicesrecognizethatthedestinationaddress(ffffffffffff)isaspecialaddressand

processtheframe.

TheoutputsampleshowstheuseofanEthernetframedestinedforabroadcast

address.

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Inthisslide,anEthernetframeiscomposedbythesourcewiththefollowingsourceand

destinationaddresses:

Src :00:13:ce:2b:6b:28Dest :01005e010101

Theframeissenttoahubthatconnectsalldevicesona4nodeLAN.Thehubbeinga

simplereplicator,sendstheframeoutonallitsportsexcepttheportwheretheframe

wasreceived(theportattachedtothesource).

Alldevicesthataremembersoftheparticulargroup(239.1.1.1)processthatmessage.

TheoutputsampleshowstheuseofanEthernetframedestinedforamulticastaddress.

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HalfduplextransmissionisthetraditionalmeansoftransportingEthernetframes.Becausedata, ,

possible.TheCSMA/CDalgorithmisusedtohandlecollisions.Ahubusessharedmediaand

supportshalfduplexonly.10BaseT,whichworksonhalfduplex,isefficient30to40%ofthetimebecauseofcollisions,andassuchtheeffectivethroughputisonly3to4Mb/s.Fullduplextransmissionhasdataforwardinginbothdirectionssimultaneously.Fulldupleximplementationsrequireapointtopointconnectionbetweenthesenderandthereceiverport.Therefore,aswitchwith8portswouldhaveeachofthe8portsconnectedtotherestoftheportsthroughadedicatedsetofwires.Thisensuresthatthereisnosharedmediumandcollisionisnotpossible.Becausedatacanbetransmittedbidirectionally,theeffectiverateofa10Mb/sfullduplextransmissionis20Mb/s(thatis,10Mb/seachway).Therefore,fullduplextransmissionsaremoreefficientthanhalfduplex.Switchesandroutersusuallysupportfullduplextransmissions.Whendevicessuchasswitchesandhubsareinterconnected,caremustbetakentoensurethatthepropertransmissionparametersaresetontheports.Forswitchtohubconnections,theswitchportmustbesettohalfduplexbecausethehubonlysupportshalfduplex.Forswitchtoswitch,switchtohost,orswitchtorouterconnections,fullduplexcanbeused.

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TheCSMA/CDaccessrulesaresummarizedbytheprotocolacronym.arr ersense ac t ernet attac e ostcont nuous y stens ortra con

themediumtodeterminewhengapsbetweenframetransmissionsoccur.

Multipleaccess

(MA)LANattachedhostscanbegintransmittinganytimethatthey

detectthatthenetworkisquiet,meaningthatnotrafficistravellingacrossthewire.Collisiondetect(CD)IftwoormoreLANattachedhostsinthesameCSMA/CDnetworkorcollisiondomainbegintransmittingatapproximatelythesametime,thebitstreamsfromthetransmittinghostswillinterfere(collide)witheachother,andbothtransmissionswillbeunreadable.Ifthathappens,eachtransmittinghostmustbecapableofdetectingthatacollisionhasoccurredbeforeithasfinishedsendingitsrespectiveframe.Eachhostmuststoptransmittingassoonasithasdetectedthecollisionandmustwaitarandomlengthoftimeasdeterminedbyabackoffalgorithmbeforeattemptingtoretransmittheframe.Inthisevent,eachtransmittinghosttransmitsa32bitjamsignal

alertingallLANattachedhostsofacollisionbeforerunningthebackoffalgorithm.TheCSMA/CDreducesthechanceofcollisionsbutdoesnotpreventthem.BothhostsAandBcoulddecidetotransmitatoncebecausenootherhostsaretransmittin amessageontheline(idleline).

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WhenhostAandhostBtransmitframesatthesametime,theybothdetectcollisions

andcorruptionofthedata.

BothhostAandhostBgenerateajamsignal,whichisreceivedbyotherhostssothattheydiscardthedatathatwasjustcorruptedbythecollision.

Arandombackofftimeristhenstartedonthetransmittinghosts.Dependingonwhose

timerexpiresfirst,eitherhostAorhostBtransmitsiftheydetectnoothertransmission

ontheline.

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FullduplexoperationisanoptionalMAClayercapabilitythatallowssimultaneoustwo

waytransmissionoverpointtopointlinks.

Fullduplextransmissioninvolvesnomediacontention,nocollisions,andnoneedtoscheduleretransmissions. Thereareexactlytwohostsconnectedonafullduplexpoint

topointlink.

Thelinkbandwidthiseffectivelydoubledbecauseeachlinkcannowsupportfullrate,

simultaneous,twowaytransmission.

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40Gand100Galsostandardisedbut notcoveredintheNRSexam

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MostnetworkscurrentlyusetheEthernetII(DIX)format.

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100BaseT4isnotverycommon today.

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10GBaseTavailable fromseveralmanufacturers.Latencyisaround1Sandhasapower

dissipationofaround6W.

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EthernetswitchesusethesourceMACaddresstodynamicallylearnwhichMAC

addressesareassociatedwithaninterface.Theswitchrecordsthisaddressinformation

intoaforwardingtableknownastheMACforwardingdatabase(FDB).WhentheswitchreceivesanEthernetframe,itrecordsthesourceMACaddressandthe

interfaceonwhichitarrived.ItlooksatthedestinationMACaddressoftheframe,

comparesittotheentriesinitsMACFDB,andtransmitstheframeoutoftheinterface

forthatMACaddress.

IfnoentryisfoundintheMACFDBforthedestination,theswitchfloodstheframeout

ofallitsinterfacesexcepttheinterfaceonwhichtheframearrived.

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AcollisiondomainisagroupofEthernetorFastEthernetdevicesinaCSMA/CDLANt atareconnecte yrepeatersan t atcompete oraccess nt enetwor . n yonedeviceinthecollisiondomainmaytransmitatanyonetime,andtheotherdevicesin

thedomainlistentothenetworktoavoiddatacollisions.AcollisiondomainissometimesreferredtoasanEthernetsegment.Abroadcastdomainisarestrictedareainwhichinformationcanbetransmittedtoalldevicesinthedomain.Morespecifically,EthernetLANsarebroadcastdomains.AnydeviceattachedtotheLANcantransmitframestoanyotherdevicebecausethemediumisasharedtransmissionsystem.Framesarenormallyaddressedtoaspecificdestinationdeviceinthenetwork.Whilealldevicesdetecttheframetransmissioninthenetwork,onlythedevicetowhichtheframeisaddressedactuallyacceptsit.Aspecialbroadcastaddressconsistingofall1sisusedtosendframestoalldevicesinthenetwork.InanIPnetwork,broadcastdomainsareseparatedbyanIProuter.Twodeviceson

separatebroadcastdomainscannotsendEthernetframesdirectlytoeachother.InsteadtheymustsendtheframetotherouterwhichthenforwardstheIPdatagramtothedestinationinanewEthernetframeonthea ro riatebroadcastdomain.

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Answers: _____collisiondomainsand_____broadcastdomains

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ALinkAggregationGroup(LAG)increasesthebandwidthavailablebetweentwonodes

bygroupinguptoeightportsintoonelogicallink.Theaggregationofmultiplephysical

linksallowsforloadsharingandoffersseamlessredundancy.Ifoneofthelinksfails,trafficisredistributedovertheremaininglinks.Uptoeightlinkscanbesupportedinone

LAG,andupto64LAGscanbeconfiguredona7x50SR/ESS.

LinkAggregationControlProtocol(LACP)isdefinedinIEEE802.3ad(Aggregationof

MultipleLinkSegments).LACPprovidesastandardizedmethodforimplementinglink

aggregationbetweendifferentmanufacturers.

NOTE:LACPmustonlybesetuponONEENDOFTHELINK.

Also,it ispossibletouseeitherDynamiccostORLAGportthresholdORboth.

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NB:Labconfigurationcanincludeonlyoneport(mightwanttosetuponenowandexpandlater)

ThisparameterdeterminesthebehaviourofaLAGwhenthenumberofavailablelinksfalls

belowtheconfiguredthresholdvalue.Twoactionscanbespecified:Option1:

configurelagportthresholdactiondownIfthenumberofavailablelinksislessthanorequaltothethresholdvalue,theLAGisdeclaredoperationallydownuntilthenumberofavailablelinksisgreaterthanthethresholdvalue.Option2:configurelagportthresholdactiondynamiccostIfthenumberofavailablelinksislessthanorequaltothethresholdvalue,dynamiccostingisuse to eterminet ea vertise LAGcost.Note:ThecostingofaLAGonlyaffectstheIGPcosting(OSPFonly)DynamicCostParameterDynamiccostcanbeenabledwiththegeneralcommandconfig>lagdynamiccost.ThisparameterenablesordisablesthedynamicIGPcostingofaLAGwhenthenumberofactivelinksisgreaterthantheportthresholdvalue.Whendynamiccostisenabledwiththiscommandandthenumberofactivelinksisgreaterthantheportthresholdvalue(07),thepathcostisdynamicallycalculatedwheneverthereischangeinthenumberofactivelinksregardlessofthespecifiedportthresholdaction.NotethatiftheportthresholdactionistodeclaretheLAG ,declareddown,evenifdynamiccostisenabled.Conversely,iftheportthresholdismetandtheactionissettodynamiccost,thenthelinkcostisdynamicallyrecalculatedevenifthegeneraldynamiccostparameterisnotconfigured.

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Inthisslide,eachphysicallinkisconfiguredwithacostof100.Thusthecostofthe

logicallinkLAG1is100/4=25andLAG2is100/3=33.

TheLAGgroupsLAG1andLAG2areconfiguredasfollows:LAG1doesnothavethedynamiccostparameterconfigured.IfonelinkinLAG1fails,

therearethreeactivelinksandtheportthresholdistwosotheportthresholdactionis

notexecuted.However,becausethedynamiccostparameterisnotenabledontheLAG,

thecostofLAG1remainsthesame(100/4=25).IfanotherlinkinLAG1fails,the

numberofactivelinksmatchestheportthresholdandtheportthresholdactionis

executed,thereforeLAG1isdeclaredoperationallydown.

LAG2doeshavethedynamiccostparameterconfigured.IfonelinkinLAG2fails,there

aretwoactivelinksandtheportthresholdistwo,sotheportthresholdactionis

executed.BecausethedynamiccostparameterisenabledontheLAG,thecostofLAG2

changesto100/2=50.

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This isafeaturerequestedbyoneofAlcatelscustomers.

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NetworksthataredesignedwithredundancyandnoSpanningTreeProtocol(STP)are,

furtherreplicateseachframeandtransmitsthemoutoneormoreportsontheswitch.

BecauseoftheLayer2loop,thetransmittedframesarereceivedbackandreplicatedagain.ThisresultsinanexponentialincreaseinLayer2trafficintheloopednetwork.Becausethereisnotimetolive(TTL)inLayer2,thisframeiscopiedandtransmittedrepeatedlyuntiltheswitchgetsoverwhelmedwithactivityandpossiblyresetsorlocksup.Considerthecasewherenotraffichasbeentransmittedontheabovenetwork.Therefore,bothSwitch1andSwitch2haveanemptyMACFDB:HostAsendsaframewithdestinationMACaddressofHostB.OnecopyoftheframeisreceivedbyHostBandprocessed.T eorigina rameisa soreceive ySwitc 1.Switc 1recor st esourceMACo HostAto eonSegment1.BecauseSwitch1doesnotknowwhereHostBis,itreplicatestheframeandsendsitouttheportconnectedtoSegment2.TheoriginalframeisalsoreceivedbySwitch2.Switch2recordsthesourceMACofHostAtobeonSegment1.BecauseSwitch2doesnotknowwhereHostBis,itreplicatestheframeandsendsitouttheportconnectedtoSegment2.Switch2receivesthereplicatedframefromSwitch1viaSegment2.Switch2removestheexistingentryforHostAintheMACFDBandrecordsthatHostAbelongstotheportattachedtoSegment2.Switch2thenreplicatestheframeandtransmitsitouttheportattachedtoSegment

.TheprocessiscontinuesindefinitelycausingabroadcaststormandMACFDBinstability.

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RedundantnetworkswithoutSTPcanalsocausedatabaseinstability.

Inthisslide,Switch1andSwitch2willmaptheMACaddressofHostAtoPort0.Later,

whenthecopyoftheframearrivesatPort1ofSwitch2,Switch2mustremoveitsoriginalentryforHostAandreplaceitwiththenewentryforHostA,mappingittoPort

1.ThisactivitycausesanunstabledatabaseasSwitch2triestokeepupwiththe

perceivedlocationofHostA.

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SpanningTreeProtocol(STP)wasinventedin1985byRadia Perlmanandwasfirst

publishedasastandardbyIEEEas802.1d.RevisionstoSTPwerepublishedin1998and

2004.RapidSpanningTreeProtocol(RSTP)wasintroducedin1998asIEEE802.1w.In2004,theIEEEincorporatedRSTPintheSpanningTreeProtocolandmadetheprevious

versionobsolete.ThisversionwaspublishedasIEEE802.1d2004.

STPisintendedtopreventloopsinanEthernetnetwork.Itdoesthisbyselectively

blockingportstoachievealoopfreetopology.ThefirstversionofSTPwasslowat

converging.EnhancementswereintroducedwithRSTPtospeedconvergenceand

convergencetimewasimprovedagainwithIEEE802.1d2004.

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SpanningTreetopologycanbethoughtofasatreethatincludesthefollowing

components:

aroot(arootbridge/switch)branches(LANSanddesignatedbridges/switches)

leaves(endnodes)

Therearenodisconnectedpartsthatareconsideredpartofthetree.Thatis,thetree

encompassesallofitsleaves.Therearenoloopsinthetree.Ifyoutraceapathfromone

leaftoanyotherleaf,thereisonlyonepossiblepath.STPorganizesandconnects

switchesintoaloopfreetopologywhileleavingnosegmentsisolated.

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TherearetwomainreasonsforthedevelopmentofVLANs:

theamountofbroadcasttraffic

increasedsecurityBroadcasttrafficincreasesindirectproportiontothenumberofstationsintheLAN.The

goalofthevirtualLAN(VLAN)istheisolationofgroupsofuserssothatonegroupisnot

interruptedbythebroadcasttrafficofanother.

VLANsalsohavethebenefitofaddedsecuritybyseparatingthenetworkintodistinct

logicalnetworks.TrafficinoneVLANisseparatedfromanotherVLANasiftheywere

physicallyseparatenetworks.IftrafficistopassfromoneVLANtoanother,itmustbe

routed.

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Onthe7750SRand7450ESSthereisnodefaultVLANforallportstojoin.Othertypes

ofswitchesmayhaveadefaultVLANforportsthatarenotassignedtoaparticular

VLAN.

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Inthisslide,VLANssubdividetheEthernetswitchintomultipleswitches.Notethatthere

arenologicalinterconnectionsbetweentheseinternalswitches.Therefore,the

broadcasttrafficthatisgeneratedbyahostinaVLANstayswithinthatVLAN,makingtheVLANitsownbroadcastdomain.BecausebroadcasttrafficforaparticularVLAN

remainswithinthatVLANsborders,interVLANorbroadcastdomaincommunication

mustoccurthroughaLayer3devicesuchasarouter.

Usually,hostsarenotVLANaware,andthereforeno802.1qconfigurationisrequiredon

thehosts.TheVLANconfigurationisdonewhentheswitchandportsareassignedona

VLANbyVLANbasis.

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ThesharingofVLANsbetweenswitchesisachievedbytheinsertionofaheaderwitha

12bitVID,whichallowsfor4094possibleVLANdestinationsforeachEthernetframe.

AVIDmustbeassignedforeachVLAN.AssigningthesameVIDtoVLANsondifferentconnectedswitchescanextendtheVLAN(broadcastdomain)acrossanetwork.

The802.1qstandardworksbyinsertinga32bitVLANheaderintotheEthernetframeof

allnetworktrafficoftheVLAN.TheVIDuses12bitsofthe32bitVLANheader.The

switchthenusestheVIDtodeterminewhichFDBitwillusetofindthedestination.After

aframereachesthedestinationswitchport,theVLANheaderisremoved.

ThisslideindicateswhichportsbelongtowhichVLAN.Thetrafficingressing aportin

oneVLANwillonlybeallowedtoegressaportonthesameswitchbelongingtothe

sameVLAN.

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VLANtrunking providesefficientinterswitch forwardingofVLANframes.Intheprevious

slide,eachVLANrequiredaseparateinterswitch connectiontoforwardframesfromone

switchtoanother.VLANtrunking allowsoneEthernetporttocarryframesfrommultipleVLANs.This

allowstheuseofonehighbandwidth port,suchasagigabitEthernetport,tocarrythe

VLANtrafficbetweenswitchesinsteadofmultiplefastEthernetports.

VLANsareseparatedwithinthetrunkbasedontheirVLANIDs(Qtags).TheFDBatthe

destinationswitchdesignatesthedestinationVLANforthetrafficontheVLANtrunk.

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TheVLANheadercanbebrokendownintotwoparts theVLANtagtypeandthetag

controlinformation.

TheVLANtagtypeisafixedvaluethatisanindicatorofaVLANtag.TheVLANtagisafixlengthof2bytes,whichisfollowedbytheoriginalEtherType describingthepayload.

Thetagcontrolinformationhasthreeparts:

Priorityvalue(Userpriority)A3bitvaluethatspecifiesaframespriority.

CFI Onebit.Asettingof0meansthattheMACaddressinformationisinitssimplest

form.Currentlynoothervalueissupported.

VIDA12bitvaluethatidentifiestheVLANthattheframebelongsto.IftheVIDis0,

thetagheadercontains onlypriorityinformation.

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ArestrictionofEthernetVLANsisthelimitednumberofVIDs.With12bitsusedto

definetheVID,thereareonly4096possibilities.BecauseVLAN0and4095arereserved,

thePEisreallyonlycapableofsupporting4094VLANs notasignificantnumberifitiscomparedwiththeexpandingratesofnetworks.

OneofthesolutionstothisrestrictionisVLANstacking,alsoknownasQinQ.VLAN

stackingallowstheserviceprovidertouseLayer2protocolstoconnectcustomersites.

Inthisslide,threecustomersareconnectedthroughacommonswitchusingVLAN

stacking.

AtthePE,theadministratorhasassignedaVLANtorepresentthecustomeronthat

port.WhenthecustomertrafficarrivesatthePEdevice,thePEswitchinsertsanother

VLANtagintheframe.ItisthissecondorstackedVLANtagthattakesthecustomer

trafficthroughtheprovidernetwork.AttheegressportofthePEequipment,thesecond

orstackVLANtagisremovedandthetrafficforwardedouttheport.

ThisallowsCustomers1,2,and3tousethesameVLANtagsintheirnetwork.Intheory,

theserviceprovidercansupport4094customers,witheachcustomersupporting4094

VLANswithintheirnetwork.

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Intheexampleonthepreviousslide,Customer1sentaframetothePEswitchwitha

VLANtagof100.ThePEswitchinsertsasecondVLANtagof20.Thistagnumber

representsCustomer1traffic.ThesecondtagkeepsCustomer1trafficseparatefromCustomer2and3trafficandgivesCustomer1theabilitytoadd4095moreassociated

VLANs.

TheVLANtagthatisinsertedbytheprovideristheVLANtagthatisusedintheprovider

network.Whentheframehasreachedtheappropriateegressport,theprovidersVLAN

tagisremovedandtheframewiththecustomersVLANtagisforwardedouttheegress

port.

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