局域网交换机中英文对照外文翻译文献Word格式.docx

局域网交换机中英文对照外文翻译文献Word格式.docx

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ThischapterintroducesmanyoftheconceptsbehindLANswitchingcommontoallswitchvendors.Thechapterbeginsbylookingathowdataarereceivedbyaswitch,followedbymechanismsusedtoswitchdataasefficientlyaspossible,andconcludeswithforwardingdatatowardtheirdestinations.TheseconceptsarenotspecifictoCiscoandarevalidwhenexaminingthecapabilitiesofanyLANswitch.

1.ReceivingData—SwitchingModes

ThefirststepinLANswitchingisreceivingtheframeorpacket,dependingonthecapabilitiesoftheswitch,fromthetransmittingdeviceorhost.SwitchesmakingforwardingdecisionsonlyatLayer2oftheOSImodelrefertodataasframes,whileswitchesmakingforwardingdecisionsatLayer3andaboverefertodataaspackets.Thischapter'

sexaminationofswitchingbeginsfromaLayer2pointofview.Dependingonthemodel,varyingamountsofeachframearestoredandexaminedbeforebeingswitched.

ThreetypesofswitchingmodeshavebeensupportedonCatalystswitches:

•Cutthrough

•Fragmentfree

•Storeandforward

Thesethreeswitchingmodesdifferinhowmuchoftheframeisreceivedandexaminedbytheswitchbeforeaforwardingdecisionismade.Thenextsectionsdescribeeachmodeindetail.

1.1Cut-ThroughMode

Switchesoperatingincut-throughmodereceiveandexamineonlythefirst6bytesofaframe.Thesefirst6bytesrepresentthedestinationMACaddressoftheframe,whichissufficientinformationtomakeaforwardingdecision.Althoughcut-throughswitchingofferstheleastlatencywhentransmittingframes,itissusceptibletotransmittingfragmentscreatedviaEthernetcollisions,runts（frameslessthan64bytes）,ordamagedframes.

1.2Fragment-FreeMode

Switchesoperatinginfragment-freemodereceiveandexaminethefirst64bytesofframe.Fragmentfreeisreferredtoas"

fastforward"

modeinsomeCiscoCatalystdocumentation.Whyexamine64bytes?

InaproperlydesignedEthernetnetwork,collisionfragmentsmustbedetectedinthefirst64bytes.

1.3Store-and-ForwardMode

Switchesoperatinginstore-and-forwardmodereceiveandexaminetheentireframe,resultinginthemosterror-freetypeofswitching.

Asswitchesutilizingfasterprocessorandapplication-specificintegratedcircuits（ASICs）wereintroduced,theneedtosupportcut-throughandfragment-freeswitchingwasnolongernecessary.Asaresult,allnewCiscoCatalystswitchesutilizestore-and-forwardswitching.

Figure2-1compareseachoftheswitchingmodes.

Figure2-1.SwitchingModes

2.SwitchingData

Regardlessofhowmanybytesofeachframeareexaminedbytheswitch,theframemusteventuallybeswitchedfromtheinputoringressporttooneormoreoutputoregressports.Aswitchfabricisageneraltermforthecommunicationchannelsusedbytheswitchtotransportframes,carryforwardingdecisioninformation,andrelaymanagementinformationthroughouttheswitch.AcomparisoncouldbemadebetweentheswitchingfabricinaCatalystswitchandatransmissiononanautomobile.Inanautomobile,thetransmissionisresponsibleforrelayingpowerfromtheenginetothewheelsofthecar.InaCatalystswitch,theswitchfabricisresponsibleforrelayingframesfromaninputoringressporttooneormoreoutputoregressports.Regardlessofmodel,wheneveranewswitchingplatformisintroduced,thedocumentationwillgenerallyrefertothe"

transmission"

astheswitchingfabric.

AlthoughavarietyoftechniqueshavebeenusedtoimplementswitchingfabricsonCiscoCatalystplatforms,twomajorarchitecturesofswitchfabricsarecommon:

•Sharedbus

•Crossbar

2.1SharedBusSwitching

Inasharedbusarchitecture,alllinemodulesintheswitchshareonedatapath.Acentralarbiterdetermineshowandwhentograntrequestsforaccesstothebusfromeachlinecard.Variousmethodsofachievingfairnesscanbeusedbythearbiterdependingontheconfigurationoftheswitch.Asharedbusarchitectureismuchlikemultiplelinesatanairportticketcounter,withonlyoneticketingagentprocessingcustomersatanygiventime.

Figure2-2illustratesaround-robinservicingofframesastheyenteraswitch.Round-robinisthesimplestmethodofservicingframesintheorderinwhichtheyarereceived.CurrentCatalystswitchingplatformssuchastheCatalyst6500supportavarietyofqualityofservice（QoS）featurestoprovidepriorityservicetospecifiedtrafficflows.

Figure2-2.Round-RobinServiceOrder

ThefollowinglistandFigure2-3illustratethebasicconceptofmovingframesfromthereceivedportoringress,tothetransmitport（s）oregressusingasharedbusarchitecture:

FramereceivedfromHost1—TheingressportontheswitchreceivestheentireframefromHost1andstoresitinareceivebuffer.Theportcheckstheframe'

sFrameCheckSequence（FCS）forerrors.Iftheframeisdefective（runt,fragment,invalidCRC,orGiant）,theportdiscardstheframeandincrementstheappropriatecounter.

Requestingaccesstothedatabus—Aheadercontaininginformationnecessarytomakeaforwardingdecisionisaddedtotheframe.Thelinecardthenrequestsaccessorpermissiontotransmittheframeontothedatabus.

Frametransmittedontothedatabus—Afterthecentralarbitergrantsaccess,theframeistransmittedontothedatabus.

Frameisreceivedbyallports—Inasharedbusarchitecture,everyframetransmittedisreceivedbyallportssimultaneously.Inaddition,theframeisreceivedbythehardwarenecessarytomakeaforwardingdecision.

Switchdetermineswhichport（s）shouldtransmittheframe—Theinformationaddedtotheframeinstep2isusedtodeterminewhichportsshouldtransmittheframe.Insomecases,frameswitheitheranunknowndestinationMACaddressorabroadcastframe,theswitchwilltransmittheframeoutallportsexcepttheoneonwhichtheframewasreceived.

Port（s）instructedtotransmit,remainingportsdiscardtheframe—Basedonthedecisioninstep5,acertainportorportsistoldtotransmittheframewhiletherestaretoldtodiscardorflushtheframe.

EgressporttransmitstheframetoHost2—Inthisexample,itisassumedthatthelocationofHost2isknowntotheswitchandonlytheportconnectingtoHost2transmitstheframe.

Oneadvantageofasharedbusarchitectureiseveryportexcepttheingressportreceivesacopyoftheframeautomatically,easilyenablingmulticastandbroadcasttrafficwithouttheneedtoreplicatetheframesforeachport.ThisexampleisgreatlysimplifiedandwillbediscussedindetailforCatalystplatformsthatutilizeasharedbusarchitectureinChapter3,"

CatalystSwitchingArchitecture."

Figure2-3.FrameFlowinaSharedBus

2.2CrossbarSwitching

Inthesharedbusarchitectureexample,thespeedoftheshareddatabusdeterminesmuchoftheoveralltraffichandlingcapacityoftheswitch.Becausethebusisshared,linecardsmustwaittheirturnstocommunicate,andthislimitsoverallbandwidth.

Asolutiontothelimitationsimposedbythesharedbusarchitectureistheimplementationofacrossbarswitchfabric,asshowninFigure2-4.Thetermcrossbarmeansdifferentthingsondifferentswitchplatforms,butessentiallyindicatesmultipledatachannelsorpathsbetweenlinecardsthatcanbeusedsimultaneously.

InthecaseoftheCiscoCatalyst5500series,oneofthefirstcrossbararchitecturesadvertisedbyCisco,threeindividual1.2-Gbpsdatabusesareimplemented.NewerCatalyst5500serieslinecardshavethenecessaryconnectorpinstoconnecttoallthreebusessimultaneously,takingadvantageof3.6Gbpsofaggregatebandwidth.LegacylinecardsfromtheCatalyst5000arestillcompatiblewiththeCatalyst5500seriesbyconnectingtoonlyoneofthethreedatabuses.AccesstoallthreebusesisrequiredbyGigabitEthernetcardsontheCatalyst5500platform.

AcrossbarfabricontheCatalyst6500seriesisenabledwiththeSwitchFabricModule（SFM）andSwitchFabricModule2（SFM2）.TheSFMprovides128Gbpsofbandwidth（256Gbpsfullduplex）tolinecardsvia16individual8-Gbpsconnectionstothecrossbarswitchfabric.TheSFM2wasintroducedtosupporttheCatalyst651313-slotchassisandincludesarchitectureoptimizationsovertheSFM.

Figure2-4.CrossbarSwitchFabric

3.BufferingData

Framesmustwaittheirturnforthecentralarbiterbeforebeingtransmittedinsharedbusarchitectures.Framescanalsopotentiallybedelayedwhencongestionoccursinacrossbarswitchfabric.Asaresult,framesmustbebuffereduntiltransmitted.Withoutaneffectivebufferingscheme,framesaremorelikelytobedroppedanytimetrafficoversubscriptionorcongestionoccurs.

Buffersgetusedwhenmoretrafficisforwardedtoaportthanitcantransmit.Reasonsforthisincludethefollowing:

•Speedmismatchbetweeningressandegressports

•Multipleinputportsfeedingasingleoutputport

•Half-duplexcollisionsonanoutputport

•Acombinationofalltheabove

Topreventframesfrombeingdropped,twocommontypesofmemorymanagementareusedwithCatalystswitches:

•Portbufferedmemory

•Sharedmemory

3.1PortBufferedMemory

Switchesutilizingportbufferedmemory,suchastheCatalyst5000,provideeachEthernetportwithacertainamountofhigh-speedmemorytobufferframesuntiltransmitted.Adisadvantageofportbufferedmemoryisthedroppingofframeswhenaportrunsoutofbuffers.Onemethodofmaximizingthebenefitsofbuffersistheuseofflexiblebuffersizes.Catalyst5000Ethernetlinecardportbuffermemoryisflexibleandcancreateframebuffersforanyframesize,makingthemostoftheavailablebuffermemory.Catalyst5000EthernetcardsthatusetheSAINTASICcontain192KBofbufferme