LANChapter03Word格式.docx
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LANChapter03Word格式.docx
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Variousswitchingmodessuchasstore-and-forward,cutthroughandothersarecomparedandcontrasted.
∙TokenRingBridging—
DifferentmethodsexistforbridgingTokenRing.Thissectiondescribesyouroptions.
∙TokenRingSwitching—
TokenRingswitchingprovidesmanyofthebenefitsfoundinEthernetswitching.ThissectiondiscussesrulesforTokenRingswitchinginaCatalystenvironment
∙EthernetorTokenRing—
Someusersareinstallingnewnetworksystemsanddonotknowwhichtouse.Thissectionprovidessomethoughtsformakingtheselection
∙MigratingTokenRingtoEthernet—
AdministratorsfrequentlyelecttoreplacelegacyTokenRingsystemswithFastEthernetswitchedsolutions.Thissectionofferssuggestionsofthingstoconsiderinsuchanupgrade.
Althoughvariousinternetworkingdevicesexistforsegmentingnetworks,Layer2LANswitchesusebridgeinternetworkingtechnologytocreatesmallercollisiondomains.Chapter2,"
SegmentingLANs,"
discussedhowbridgessegmentcollisiondomains.Butbridgesdofarmorethansegmentcollisiondomains:
theyprotectnetworksfromunwantedunicasttrafficandeliminateactiveloopswhichotherwiseinhibitnetworkoperations.HowtheydothisdiffersforEthernetandTokenRingnetworks.EthernetemploystransparentbridgingtoforwardtrafficandSpanningTreetocontrolloops.TokenRingtypicallyusesaprocesscalledsource-routebridging.Thischapterdescribestransparentbridging,source-routebridging(alongwithsomevariations),andLayer2LANswitching.Chapter6coversSpanningTreeforEthernet.
TransparentBridging
AsdiscussedinChapter2,networksaresegmentedtoprovidemorebandwidthperuser.Bridgesprovidemoreuserbandwidthbyreducingthenumberofdevicescontendingforthesegmentbandwidth.Butbridgesalsoprovideadditionalbandwidthbycontrollingdataflowinanetwork.Bridgesforwardtrafficonlytotheinterface(s)thatneedtoreceivethetraffic.Inthecaseofknownunicasttraffic,bridgesforwardthetraffictoasingleportratherthantoallports.Whyconsumebandwidthonasegmentwheretheintendeddestinationdoesnotexist?
Transparentbridging,definedinIEEE802.1ddocuments,describesfivebridgingprocessesfordeterminingwhattodowithaframe.Theprocessesareasfollows:
1.Learning
2.Flooding
3.Filtering
4.Forwarding
5.Aging
Figure3-1illustratesthefiveprocessesinvolvedintransparentbridging.
Figure3-1TransparentBridgeFlowChart
Whenaframeentersthetransparentbridge,thebridgeaddsthesourceEthernetMACaddress(SA)andsourceporttoitsbridgingtable.Ifthesourceaddressalreadyexistsinthetable,thebridgeupdatestheagingtimer.ThebridgeexaminesthedestinationMACaddress(DA).IftheDAisabroadcast,multicast,orunknownunicast,thebridgefloodstheframeoutallbridgeportsintheSpanningTreeforwardingstate,exceptforthesourceport.Ifthedestinationaddressandsourceaddressareonthesameinterface,thebridgediscards(filters)theframe.Otherwise,thebridgeforwardstheframeouttheinterfacewherethedestinationisknowninitsbridgingtable.
Thesectionsthatfollowaddressingreaterdetaileachofthefivetransparentbridgingprocesses.
Learning
Eachbridgehasatablethatrecordsalloftheworkstationsthatthebridgeknowsaboutoneveryinterface.Specifically,thebridgerecordsthesourceMACaddressandthesourceportinthetablewheneverthebridgeseesaframefromadevice.Thisisthebridgelearningprocess.Bridgeslearnonlyunicastsourceaddresses.Astationnevergeneratesaframewithabroadcastormulticastsourceaddress.BridgeslearnsourceMACaddressesinordertointelligentlysenddatatoappropriatedestinationsegments.Whenthebridgereceivesaframe,itreferencesthetabletodetermineonwhatportthedestinationMACaddressexists.Thebridgeusestheinformationinthetabletoeitherfilterthetraffic(ifthesourceanddestinationareonthesameinterface)ortosendtheframeoutoftheappropriateinterface(s).
Butwhenabridgeisfirstturnedon,thetablecontainsnoentries.AssumethatthebridgesinFigure3-2wereallrecentlypowered"
ON,"
andnostationhadyettransmitted.Therefore,thetablesinallfourbridgesareempty.NowassumethatStation1transmitsaunicastframetoStation2.Allthestationsonthatsegment,includingthebridge,receivetheframebecauseofthesharedmedianatureofthesegment.BridgeAlearnsthatStation1existsoffofportA.1bylookingatthesourceaddressinthedatalinkframeheader.BridgeAentersthesourceMACaddressandbridgeportinthetable.
Figure3-2SampleBridgedNetwork
Flooding
ContinuingwithFigure3-2,whenStation1transmits,BridgeAalsolooksatthedestinationaddressinthedatalinkheadertoseeifithasanentryinthetable.Atthispoint,BridgeAonlyknowsaboutStation1.Whenabridgereceivesaunicastframe(aframetargetingasingledestination),notableentryexistsfortheDA,thebridgereceivesanunknownunicastframe.Thebridgingrulesstatethatabridgemustsendanunknownunicastframeoutallforwardinginterfacesexceptforthesourceinterface.Thisisknownasflooding.Therefore,BridgeAfloodstheframeoutallinterfaces,eventhoughStations1and2areonthesameinterface.BridgeBreceivestheframeandgoesthroughthesameprocessasBridgeAoflearningandflooding.BridgeBfloodstheframetoBridgesCandD,andtheylearnandflood.NowthebridgingtableslooklikeTable3-1.ThebridgesdonotknowaboutStation2becauseitdidnotyettransmit.
Table
3-1.BridgingTableafterFlooding
BridgePort
A.1
A.2
B.1
B.2
B.3
C.1
C.2
D.1
MACAddress
1
StillconsideringFigure3-2,allthebridgesinthenetworkhaveanentryforStation1associatedwithaninterface,pointingtowardStation1.Thebridgetablesindicatetherelativelocationofastationtotheport.ExaminingBridgeC'
stable,anentryforStation1isassociatedwithportC.1.ThisdoesnotmeanStation1directlyattachestoC.1.ItmerelyreflectsthatBridgeCheardfromStation1onthisport.
Inadditiontofloodingunknownunicastframes,legacybridgesfloodtwootherframetypes:
broadcastandmulticast.Manymultimedianetworkapplicationsgeneratebroadcastormulticastframesthatpropagatethroughoutabridgednetwork(broadcastdomain).Asthenumberofparticipantsinmultimediaservicesincreases,morebroadcast/multicastframesconsumenetworkbandwidth.Chapter13,"
MulticastandBroadcastServices,"
discusseswaysofcontrollingmulticastandbroadcasttrafficflowsinaCatalyst-basednetwork.
Filtering
WhathappenswhenStation2inFigure3-2respondstoStation1?
AllstationsonthesegmentoffportA.1,includingBridgeA,receivetheframe.BridgeAlearnsaboutthepresenceofStation2andaddsitsMACaddresstothebridgetablealongwiththeportidentifier(A.1).BridgeAalsolooksatthedestinationMACaddresstodeterminewheretosendtheframe.BridgeAknowsStation1andStation2existonthesameport.Itconcludesthatitdoesnotneedtosendtheframeanywhere.Therefore,BridgeAfilterstheframe.Filteringoccurswhenthesourceanddestinationresideonthesameinterface.BridgeAcouldsendtheframeoutotherinterfaces,butbecausethiswastesbandwidthontheothersegments,thebridgingalgorithmspecifiestodiscardtheframe.NotethatonlyBridgeAknowsabouttheexistenceofStation2becausenoframefromthisstationevercrossedthebridge.
Forwarding
IfinFigure3-2,Station2sendsaframetoStation6,thebridgesfloodtheframebecausenoentryexistsforStation6.AllthebridgeslearnStation2'
sMACaddressandrelativelocation.WhenStation6respondstoStation2,BridgeDexaminesitsbridgingtableandseesthattoreachStation2,itmustforwardtheframeoutinterfaceD.1.Abridgeforwardsaframewhenthedestinationaddressisaknownunicastaddress(ithasanentryinthebridgingtable)andthesourceanddestinationareondifferentinterfaces.TheframereachesBridgeB,whichforwardsitoutinterfaceB.1.BridgeAreceivestheframeandforwardsitoutA.1.OnlyBridgesA,B,andDlearnaboutStation6.Table3-2showsthecurrentbridgetables.
3-2.BridgingTableafterForwarding
D.2
2
2*
6
*.B.1,C.1,andD.1didnotlearnaboutStation2untilStation2transmittedtoStation6.
Aging
Whenabridgelearnsasourceaddress,ittimestampstheentry.Everytimethebridgeseesaframefromthatsource,thebridgeupdatesthetimestamp.Ifthebridgedoesnothearfromthatsourcebeforeanagingtimerexpires,thebridgeremovestheentryfromthetable.Thenetworkadministratorcanmodifytheagingtimerfromthedefaultoffiveminutes.
Whyremoveanentry?
Bridgeshaveafiniteamountofmemory,limitingthenumberofaddressesitcanrememberinitsbridgingtables.Forexample,higherendbridgescanrememberupwardsof16,000addresses,whilesomeofthelower-endunitsmayrememberasfewas4,096.Butwhathappensifall16,000spacesarefullinabridge,butthereare16,001devices?
Thebridgefloodsallframesfromstation16,001untilanopeninginthebridgetableallowsthebridgetolearnaboutthestation.Entriesbecomeavailablewh
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