LANChapter03Word格式.docx

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LANChapter03Word格式.docx

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LANChapter03Word格式.docx

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

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

*.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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