Featuresandadvantages
AllsitesoftheswitchedLANareconnectedtoaswitchinghuborLANswitch.SwitchinghubsorLANswitcheshaveswitchingfunctions.Theircharacteristicsare:allportsarenotconnectedatordinarytimes.Whentheworkstationneedstocommunicate,theswitchinghuborLANswitchcanconnecttomanyportsatthesametime,sothateachpairofportscancommunicateexclusively.Themediatransfersdatawithoutconflicts,andtheconnectionisdisconnectedafterthecommunicationiscompleted.Sincethecommoncommunicationmediumiseliminated,eachsiteusesalinkindependently,thereisnoconflictproblem,andtheaveragedatatransmissionrateofuserscanbeincreased,thatis,thecapacitycanbeexpanded.TheadvantagesoftheswitchedLAN:(1)Itadoptsastartopology,whichiseasytoexpand,andthebandwidthofeachuserisnotreducedduetotheincreaseofinterconnecteddevices.(2)Sincethecommoncommunicationmediumiseliminated,eachsiteusesalinkindependently,thereisnoconflictproblem,andtheaveragedatatransmissionspeedofuserscanbeimproved.TheswitchedLANisastartopologybothphysicallyandlogically,andmultipleswitchinghubscanbeconnectedinseriestoformamulti-levelstarstructure.
Function
SwitchedLANcanprovideuserswithsomefunctionsthatcannotberealizedbysharedLAN,mainlyincludingthefollowingaspects:
(1)IsolateconflictsThedomainisinasharedEthernet,usingtheCSMA/CDalgorithmformediaaccesscontrol.Iftwoormorestationssimultaneouslydetectthatthechannelisidleandthereareframesreadytobesent,theywillcollide.Agroupofsitesthatcompeteforchannelaccessiscalledacollisiondomain.Obviously,stationsinthesamecollisiondomaincompeteforchannels,whichwillleadtocollisionsandback-offs.However,stationsindifferentconflictdomainswillnotcompeteforthecommonchannel,andtheywillnotconflict.InaswitchedLAN,eachswitchportcorrespondstoacollisiondomain,andtheportistheendofthecollisiondomain.Becausetheswitchhasaswitchingfunction,therewillbenoconflictbetweensiteswithdifferentports.Ifonlyonecomputersiteisconnectedtoeachport,therewillbenoconflictsbetweenanypairofsites.IfaportisconnectedtoasharedLAN,thentherewillbeconflictsbetweenallsitesontheport,buttherewillbenoconflictsbetweenthesitesonthisportandthesitesonotherportsoftheswitch.Therefore,theswitchisolatestheconflictdomainofeachport.
(2)ExtendthedistanceTheswitchcanextendthedistanceoftheLAN.EachswitchportcanbeconnectedtoadifferentLAN.Therefore,eachportcanreachthemaximumdistancerequiredbydifferentLANtechnologies,regardlessofthelengthoftheLANconnectedtootherswitchports.
(3)IncreasethetotalcapacityInasharedLAN,thecapacityissharedbyallaccessdevices.InaswitchedLAN,sinceeachportoftheswitchhasadedicatedcapacity,thetotalcapacityoftheswitchedLANincreaseswiththenumberofportsoftheswitch.Therefore,thedatatransmissioncapacityprovidedbytheswitchismuchlargerthanthatofthesharedLAN.
(4)DatarateflexibilityForsharedLANs,differentLANsusedifferentdatarates,butalldevicesconnectedtothesamesharedLANmustusethesamedatarate.ForswitchedLANs,eachportoftheswitchcanusedifferentdatarates,sositescanbedeployedatdifferentdatarates,whichisveryflexible.
Basicprinciples
ThecoredeviceofaswitchedLANisaLANswitch,whichcanestablishmultipleconcurrentconnectionsbetweenitsmultipleports.AtypicalswitchedLANisaswitchedEthernet(switchedEthernet),anditscorecomponentisanEthernetswitch(Ethernetswitch).TheEthernetswitchcanhavemultipleports,andeachportcanbeindividuallyconnectedtoanode,oritcanbeconnectedtoasharedmedium-typeEthernethub.1.ClassificationofLANswitches:Accordingtothedifferentfunctionsperformed,LANswitchescanbedividedintotwotypes.(1)Layer2switching:Performbridgingfunction,whichforwardsdataaccordingtoMACaddress.Theswitchingspeedisfast,butthecontrolfunctionisweakandthereisnoroutingfunction.(2)Three-layerexchange:ItforwardsdataaccordingtotheIPaddressandhasaroutingfunction.Layer3switchingisanorganiccombinationofLayer2switchingandroutingfunctions.
Technicalfeatures
(1)Lowexchangedelay.Thisisthemainfeatureoftheswitch.Fromtheperspectiveofthemagnitudeofthetransmissiondelay,iftheswitchistensofmicroseconds,thebridgeishundredsofmicroseconds,andtherouteristhousandsofmicroseconds.
(2)Supportdifferenttransmissionratesandworkingmodes.Theportoftheswitchcansupportdifferenttransmissionrates,suchas10Mbps,100Mbps,etc.Theportcansupporttwoworkingmodes,full-duplexandhalf-duplex.
(3)Hightransmissionbroadband.
(4)Supportvirtuallocalareanetworkservice.SwitchedLANisthebasisofvirtuallocalareanetworks,andcurrentswitchesbasicallyonlysupportvirtuallocalareanetworks.
Switch
Internalstructure
Theinternalstructureoftheswitchdeterminestheperformanceoftheswitch.Therearefourmaininternalstructuresused:
(1)Sharedmemorystructure:Thesharedmemorystructureisthattheframeisdirectlytransmittedfromthememorytotheoutputport.Thereisnoneedtousethebackplanebusconnectionbetweenthemodules,andthecentralswitchingengineisusedtoprovidehigh-performanceconnectionsforallports.ThecentralswitchingengineCheckeachinputpackettodeterminetheroute.Thismethodiseasytoimplement,butrequiresalargememorycapacityandhighmanagementcosts.Andbecauseittakestimetoaccessthestorage,itisimpossibletoachievewire-speedswitchingbetweenalargenumberofports,soitismoresuitableforsmallsystemswitches.
(2)Crossbusstructure:Thecrossbusstructureestablishesadirectpoint-to-pointconnectionbetweenports,andeachmoduleisdirectlyconnectedtoanyothermodule.Eachmodulehandlestheconnectionproblembyitself.Thereisnoneedforacentralexchangedisplaymoduleforcentralizedcontrol.Thisstructureissuitableforsingle-pointtransmission,andtherearecertainproblemswithmulti-pointtransmission.
(3)Hybridcrossbusstructure:Thehybridcrossbusstructureisimprovedonthebasisofthecrossbusstructure.Itdividestheintegratedcrossbusmatrixintosmallcrossmatrixes,whichareconnectedthroughahigh-performancebus.Theadvantageisthatitreducesthenumberofcrossoverbuses,reducescosts,andreducesbuscontention.Butthebusconnectingthecrossmatrixmaybecalledanewperformancebottleneck.
(4)Ringbusstructure:Thisstructuresupportsupto4switchingenginesinaringandallowsswitchingmatrixinterconnectionofdifferentspeeds,andtheringsareconnectedbyswitchingengines.Differentfromthepreviousstructures,thisstructurehasanindependentcontrolbus,whichisusedtocollectbusstatus,processrouting,flowcontrolandcleanupthedatabus.Thebiggestadvantageoftheringbusstructureisstrongexpansioncapabilityandlowcost.Becausetheringstructureisused,itiseasytogatherbandwidth.Whenthenumberofportsincreases,thebandwidthincreasesaccordingly.Inaddition,iteffectivelyavoidsthebusbottleneckcausedbysystemexpansion.
Exchangemethod
TheexchangemethodoftheswitchGenerally,theswitchmainlyrealizestheexchangethroughthefollowing4ways.
(1)Straight-through:Inthismode,theswitchonlyneedstoknowthedestinationMACaddressoftheframetosuccessfullyforwardtheframetothedestination.Aftertheswitchhasreadenoughinformationintheframeandcanidentifythedestinationaddress,itwillimmediatelysendtheframetothedestinationport.Theadvantageofthestraight-throughtypeisthatsincenostorageisrequired,thedelayisverysmallandtheexchangeisveryfast.Butthedisadvantageisthatbecausethereisnobuffer,thecontentofthedatapacketisnotsavedbytheEthernetswitch,soitisimpossibletocheckwhetherthetransmitteddatapacketiswrong,itcannotprovideerrordetectioncapability,anditiseasytolosepackets.
(2)Store-and-forward:Thestore-and-forwardmethodistostorethedatapacketoftheinputportfirst,andthenperformCRCcheck,andthentakeoutthedestinationaddressofthedatapacketafterprocessingtheerrorpacket,andfindtheMACaddressThetableisconvertedintoanoutputporttosendoutpackets.Becausethismethodcanperformerrordetectiononthedatapacketsenteringtheswitch,andgreatlyreducetheinvalidframesinthenetwork,itcaneffectivelyimprovethenetworkperformance.Butthedisadvantageisthatthedataprocessingdelayislargeduetotheneedtostoreandforward.However,withthedecreaseofASICandtheincreaseofprocessorspeed,manynewswitchescancompletetheentireframeinspectioninashorttime,sothisThisexchangemethodiswidelyused.
(3)Fragmentisolation:Fragmentisolationisacombinationoftheabovetwotechnologies.Itcheckswhetherthelengthofthedatapacketisenoughfor64B,ifitislessthanthisvalue,itmeansitisafakepacket,thendiscardthepacket;ifitisgreaterthanthisvalue,thensendthepacket.Thismethodalsocannotprovidedataverification.Itsdataprocessingspeedisfasterthanstore-and-forward,butslowerthanstraight-through.
(4)Intelligentswitchingmode:Thesmartswitchingmodecombinestheadvantagesofbothstraight-throughandstore-and-forward.Wheneverpossible,theswitchalwaysadoptsthestraight-throughmode,butoncethenetworkerrorrateexceedsthepresetthreshold,theswitchwilladoptthestore-and-forwardmode.Whenthenetworkerrorratedrops,thestraight-throughmoderestarts.
Features
1.LowswitchingdelayThisisthemainfeatureoftheLANswitch.Fromtheperspectiveofthemagnitudeofthetransmissiondelaytime,iftheLANswitchistensofμs,thenthebridgeisAfewhundredμs,whiletherouterisseveralthousandμs.
2.SupportingdifferenttransmissionratesandworkingmodesTheportsofLANswitchescanbedesignedtosupportdifferenttransmissionrates,suchas10Mb/sports,100Mb/sports,and1000Mb/sports.port.Atthesametime,theportcanalsobedesignedtosupporttwoworkingmodes,half-duplexandfull-duplex.
3.SupportvirtuallocalareanetworkserviceSwitchedlocalareanetworkisthebasisofvirtuallocalareanetwork,andEthernetswitchescanbasicallysupportvirtuallocalareanetworkservice.
4.Hightransmissionbandwidth
Communicationmethod
1.Theconceptoffull-duplexLAN,allsharedLANsarehalf-duplex,thatisThechannelcanonlytransmitdatainonedirectionatanytime,eithersendingdataorreceivingdata,notboth.BecauseallusersinasharedLANrelyonasinglesharedmedium,itistechnicallyimpossibletosendandreceivedataatthesametime.Eachsiteofafull-duplexLANcansendandreceivedataatthesametime,onepairofwiresisusedtosenddata,andtheotherpairofwiresisusedtoreceivedata.Switchingtechnologyisanecessaryprerequisiteforfull-duplexEthernet,becausefull-duplexrequiresapoint-to-pointconnectionwithonlytwostations.ButonethingtonoteisthattheswitchedLANisnotautomaticallyfull-duplexoperation.Onlywhenafull-duplexportissetintheswitchandsomecorrespondingimprovementsaremade,theswitchedLANisafull-duplexLAN.
2.Theadvantageofafull-duplexlocalareanetworkisthatitcantheoreticallydoublethetransmissionspeedduetosimultaneoustransmissionandreception.Forexample,thelinkrateof10BASE-Ttwistedpairthatworksinfullduplexmodecanreach20Mbit/s.Thelengthofthenetworksegmentisnolongerrestrictedbythetimingrequirementsofthesharedmediumhalf-duplexLAN,itisonlyrestrictedbythesignaltransmissioncapabilityofthemediumsystemitself.Forexample,inhalf-duplexmode,thelengthofthe100BASE-FXopticalfibernetworksegmentislimitedto412m,whilethelengthofthesamemediasysteminfull-duplexmodecanreach2000m.3.Full-duplexLANstandardIEEEandthe802.3xfull-duplexLANstandardwereformallyformulatedinMarch1997.Thestandardspecifiestheuseoffull-duplexoperationandafull-duplexflowcontrolmechanism.TheIEEE802.3xstandardstipulatesthatfull-duplexoperationshouldmeetthefollowingrequirements:thephysicalmediummustsupportsimultaneoustransmissionandreceptionofsignalswithoutinterference;afull-duplexpoint-to-pointlinkmustconnecttwosites;bothsitesontheLANcanandhaveConfiguretousefullduplexmode.ThismeansthattwoLANinterfacesmustbeabletosendandreceiveframesatthesametime.
VLAN
Definition
VLANisroughlyequivalenttoabroadcastdomain,thatis,VLANsimulatesagroupofterminaldevices,althoughtheyarelocatedondifferentphysicalnetworksegments,Butarenotrestrictedbyphysicallocation,andcommunicatewitheachotherasiftheyareinthesamelocalareanetwork.VLANisderivedfromtheconceptoftraditionalLAN,whichisbasicallythesameastraditionalLANinfunctionandoperation,andprovidesinterconnectionanddatatransmissionofterminalsystemswithinacertainrange.ThemaindifferencebetweenitandthetraditionalLANistheword"virtual",thatis,thecompositionofthenetworkisdifferentfromthatofthetraditionalLAN,whichalsoleadstothedifferenceinperformance.
Classification
(1)PortVLAN,portVLANisdividedintosingleswitchportdefinitionVLANandmulti-switchportdefinitionVLAN.
(2)MACVLAN,aVLANdefinedbasedonMACcanberegardedasauser-basedVLAN.ThiskindofVLANrequiresthatallusersmustbeconfiguredinatleastoneVLANintheinitialstage.Theinitialconfigurationiscompletedmanually,andthenuserscanbeautomaticallytracked.
Mainadvantages
(1)Reducethecostofworkstationmovementandchange.
(2)VLANandswitchingtechnologymakeeachnetworksegmentcontainfewerusers,whilethebroadcastdomainhasexpandedto1000ormoreusers.
Networkingtechnology
1.Group-levelswitchingnetwork:Atypicalgroupcanuseabasic10Mbit/sEthernetswitch,withsome100Mbit/sports,whichcanbeusedwithOneormorelocalfileserverconnections.
2.Department-levelswitchingnetwork:Severalgroupswitchingnetworksarecombinedtoformadepartment-levelswitchingnetwork,whichisgenerallyatwo-levelswitchingnetwork.Thefirst-levelorlower-levelswitchesspecificallysupportspecificgroups,includinglocalservers.Oneorseveralswitchesattheupperlevelareusedtoconnectthedepartmentserversofthegroupswitch.Groupusers'accesstodepartmentalserversneedstocrossthegroupboundary,thatis,throughthebackboneswitch.
3.Enterprise-levelswitchingnetwork:Ifyouareondepartment-levelswitches,youneedtouserouterstoconnectgeographicallydisperseddepartmentstoformanenterprise-levelswitchingnetwork.
Architecture
LargeLANsarealwaysconnectedbymultipleLANsthroughvariousnetworkinterconnectiondevices,suchasbridges,routersorswitches.BecausetheincreasingrequirementsforLANbandwidthmustbeunderthefixed10Mbpsor16MbpsbandwidthlimitofEthernetorTokenRing,thenumberofdifferentLANsegmentsinatypicalLANdesignisrapidlyincreasing.SwitchedLAN,asatechnologythatcanincreasethecapacityoftheLANbyaddingnetworksegments,hasquicklyestablisheditsownposition.ThisisbecausetheLANswitchcanprovidehigh-qualitymessagetransmissionservicesinmultiplenetworksegmentsatalowercost.Thisisjustlikeroutersinthepast,asinterconnectingdevicesconnectingLANsegmentshavelargelyreplacedinterconnectionbridges,andswitchestendtoreplaceroutersinLANs.
TheroleofroutinginaswitchedLAN:BeforeunderstandingtherespectiverolesofswitchingandroutinginaLAN,youshouldfirstunderstandthedifferencebetweenthesetwotechnologies.LANswitchesareabitlikebridges.UsuallytheyinterconnectthesametypesofLANsegments,suchasallEthernetsegmentsortokenringsegments.Theytransmitinformationtransparentlybetweenports.Takethetokenringnetworkasanexample,usingsourcerouting.Transparentswitchesareinvisibletoendstations.TheylearnbycheckingalltheinformationpacketstransmittedtotheirportsintheLANsegment,therebyknowingthelocationofeachstation,andaccordingtothedestinationnetworkaddressineachinformationpacket.Thepacketissenttotheappropriateport.Thisalsomeansthattheiroperationisindependentofthecommunicationprotocolwiththeendstation,whetheritistheTCP/IPprotocol,orNovellIPX,NETBIOSorIBM'sSNAprotocol.Thesourceroutingswitchofthetokenringnetworkisdifferentfromthetransparentswitchonlyinthatthesourceroutingswitchsendstheinformationpackettothecorrespondingportbasedontheinformationinsertedintoeachinformationpacketbytheendstation,whichisalsoindependentBasedontheunderlyingnetworkprotocol.
Butinsomecases,switchescanbeusedtointerconnectdifferenttypesoflocalareanetworks.Forexample,someswitchescaninterconnectFDDIbackbonenetworksandEthernetsegments.Inthiscase,theswitchjustdoessomesimpleconversionworkbetweentheEthernetandFDDIframes,thusfollowingthetransparencyprincipleoftheoppositestation.Ontheotherhand,routersaredesignedtohavetheabilitytotransmitanytypeofnetworkinformationpackettoanyothertypeofnetwork,andtheyareopaquetotheendstation:infact,whenanEthernetendstationwantsthestationattheotherendoftherouterWhencommunicating,itonlyaddressesthecorrespondingrouter,notthedestinationsite.WhenarouterreceivesapacketfromanEthernetsegmenttoanothernetworksegment,theroutertakesouttheheaderofthepacket,checksthedestinationaddressintheheader,andthenqueriesthecorrespondingtablebasedontheinformationtodeterminethedestinationWhetherthesiteislocatedinadirectlyconnectedLANsegment,otherwise,theinformationpacketshouldbesenttoanotherrouter.Aftermakingthecorrespondingdecision,therouterwilladdanewheadertotheinformationpacketandsenditout.
Inordertodeterminewhichporttheinformationpacketisforwardedto,routersneedtomaintaincomplexlookuptables.Thesetablesareconstructedbyeachroutercooperatingwithotherroutersinthenetwork.TheserouterspassthroughthisTheroutingstatusinformationofthenetwork,theprotocolsandprocessesinvolvedinroutingselectionarecomplicated,requirealotofcalculations,andtakeupmemory.Allinall,themostsignificantdifferencebetweenswitchingandroutinginalocalareanetworkisthatitrequiresmuchmorecomplicatedprocessingforinformationpacketstopassthrougharouterthantopassthroughaswitch.Therefore,underthepremiseofachievingthesamelevelofperformance,thecostoftherouterismuchmorethanthatoftheswitch,andittakeslesstimeforapackettopassthroughtheswitchthantopassthroughtherouter,sotheswitchprovidesashorterdelay;butontheotherhand,Theprocessingpoweroftheroutercanbeusedtoprovideagreaterdegreeofcontrolthantheswitch.
ThegoalofnetworkdesignSomecommondesigngoalsofswitchedLAN:1.Obtainhigherprocessingpoweratareasonablecost.2.Lowerend-to-enddelay.3.Ithastheflexibilitytoadjustthecommunicationmode.4.Easytoconfigureandinstall.5.Minimizemanagementburden.6.Theeffectivecontrolswitchingtechnologyfornetworkresourceaccessisthedominanttechnology,andthelocalareanetworkdesignwhereroutingtechnologyplaysanimportantbutsmallerrolecanbestmeetmostoftheabovedesigngoals.Ahighpercentageofswitchingtechnologiesinthismixisusuallysatisfactorybecauseswitchingtechnologiescanprovidegreatercommunicationprocessingcapabilitiesatalowercostthanroutingtechnologies,andswitchesareeasiertoinstall,configure,andmanage.
TheroleofroutinginswitchedLANs.InswitchedLANs,therearemainlyfourbasicfunctionsperformedbyrouters.AclearunderstandingofthemwillhelptounderstandthatroutingisinswitchedLANs.Thesefourfunctionsareasfollows:1.SplittheswitchedLANintomultiplebroadcastdomainsandconnectthesedomainstogether.2.Thetransmissionofinformationpacketsbetweendifferentsubnets.3.Asatechnologyforinterconnectingdifferentlocalareanetworks.4.Provideamechanismforsecureaccesstoresourcesonthelocalareanetwork.Ofcourse,therouterperformsmorethanthesefunctions.Whenconnectingthelocalareanetworktothewideareanetwork,therouterundertakestheconversionofmanyprotocols,suchasfromthelocalareanetworkprotocoltothepoint-to-pointprotocol(PPP)fordedicatedlineortelephonelineconnection,orframerelay.
(1)DividetheswitchedLANintomultiplebroadcastdomains.SomeLANtechnologies(suchasEthernetandTokenRing)provideforanysitetosendapackettoallothersitesintheLANTheabilityofthisisalsocalledbroadcasting.Almostallthenetworkprotocolsofthelocalareanetworkusebroadcasttorealizethemechanismofoperationandmanagement.Forexample,enablingtheclienttolocatetheserver,allowingthedisseminationofinformationaboutavailablenetworkresources,andsoon.Generallyspeaking,themoresitesconnectedtothesamelocalareanetwork,thegreatertheamountofbroadcasttrafficgenerated.Thissituationisstilltrueforlarge-scaleLANsformedbyconnectingmultipleLANsegmentsthroughbridgesorswitches.
(2)Theamountofbroadcasttrafficinalocalareanetworkisnotonlydeterminedbythenumberofsitesconnectedtothelocalareanetwork,butalsoaffectedbymanyotherfactors,suchasthenumberofserversandroutersonthelocalareanetwork.Thenumber,thetypeofprotocolused,thefrequencywithwhichtheuseractivatesandterminatesthenetworkapplication,etc.Atthesametime,theobservablebroadcastcharacteristicsinthetokenringnetworkaredifferentfromthatoftheEthernet,becausethetokenringnetworkusesasourceroutedetectionframe(SourceRouteEXPloreFrames),whichwillfacemultiplerouteswhenpassingthroughabridgednetwork.Thechoicewillcopyitself.BecausetherearemanyfactorsthataffectLANbroadcasttraffic,itisdifficulttogiveageneralmeasurementindex.However,theactualnetworkmeasurementshowsthatevenacommonbridgeorswitchisusedtoconnectalocalareanetworkwithhundredsoreventhousandsofnodes.
Theaveragebroadcastcommunicationvolumegenerallydoesnotexceed10-30informationpacketspersecond,andthereareonly100-150informationpacketspersecondintheoccasionalpeakperiod.And30broadcastpacketspersecondmeansthattheyoccupyabouttwoandahalfthousandthsoftheEthernetchannel,(hereweassumethattheaveragelengthofbroadcastpacketsis100bytes).Therefore,theimpactofthebroadcaststreamontheperformanceoftheentirenetworkisnegligible.AlthoughthebroadcaststreamontheLANhaslittleeffectonnetworkperformance,thesamesituationdoesnotapplytoWANconnections.Inthiscase,thebroadcastcommunicationstreamwilloccupyaconsiderablepartofthevaluableWANbandwidth,andtherouterplaysaroleinminimizingtheimpactofbroadcastcommunicationinthisenvironment.Thecurrenttrendinthetypesandusageofnetworkprotocolsandsoftwareis:tendtoreducethebroadcastcommunicationtrafficinthelocalareanetwork.
(3)BroadcastStorm(BroadcastStorm)Systemadministratorswithmanyyearsofnetworkmanagementexperiencemaybeawareofbroadcaststorms.Inalargenetwork,ahigh-levelbroadcastcommunicationstreammaytemporarilybombardacertainpartofthenetwork,causingthesitetoloseitsconnectionwiththeserver,sowhenthesesitestrytoreestablishtheirconnection,ittriggersmorebroadcastcommunicationstreams,thuscausingTherippleeffectisthebroadcaststorm.Eventually,therapidlygrowingbroadcasttrafficflowwillfloodtheentirenetworkandparalyzetheentirenetwork.Theroutercansolvethebroadcaststormproblemwell.Thebroadcastpacketsentbytheclienttofindtheserverisinterceptedattherouter.Forwardforwardingbytherouter.Therefore,therouterprovidesatypeoffirewallforbroadcastpackets.Thissuppressesthechainreactionthatmaytriggerabroadcaststorm.ThefearofbroadcaststormshascausedtheroutertobethecenteroftheLANdesign.Laterwewillexplaintherouter-centricnetworkstructure.Thereisnodoubtthatintoday'slargeLANsinterconnectedbybridges,broadcaststormswillcauseveryseriousnetworkservicelossproblems.However,theemergenceofthisproblemismainlyduetothreefactsthathavenotbeenpaidenoughattentionsofar:
Useremotebridgestoconnectexternalnetworkpointsthroughlow-speeddedicatedlines.ThisoriginalremoteLANbridgehaslittleornobroadcastpacketfilteringcapabilities.Therefore,thebroadcastcommunicationtrafficthatoriginallyoccupiedanegligiblebandwidthinthe10MbpsEthernetmaysoonbombthe64Kbpsline.Theresultoflosingconnectionsbetweensitescaneasilytriggerabroadcaststorm.Inpractice,routersareoftenusedtosupportlow-speedlinestoconnecttoremotesites,androutersareusedtopreventremotelinesfrombeingbombardedbybroadcastpackets.ThecharacteristicswhentheendstationimplementstheIPprotocolstackarealsopronetocauseabroadcaststorm.ManyearlywaystoimplementtheIPprotocolstackaredescribedintheinformationaboutIP,allofwhichmaycauseabroadcaststorm.Forexample,intheearlyversionofBerkeleyUNIX,thesitewillcontinuetoforwardapacketwithawrongIP,andthesitemaysendoutICMPerrormessagesforspecificbroadcastpackets.ThecurrentversionoftheIPimplementationhaseliminatedthisproblem.
Poorimplementationofthenetworkinterfaceandprotocolstackoftheendstation.Duetohistoricalreasons,insufficientprocessingpower,insufficientbuffermemory,andimmaturesoftwareimplementationoftheprotocolstackhavecausedexcessivesensitivitytothebroadcastcommunicationflowinthelocalareanetwork.Ifinthecaseofrelativelylow-levelbroadcasttraffic,theLANinterfacebecomescongested,theconnectionmaybelost,andthesite'seffortstore-establishtheconnectionformtheconditionsthattriggerabroadcaststorm.Aftermorethantenyearsoftechnologicaldevelopment,theLANinterfacecanhandleveryhighbroadcaststreams.Thelowerlimitofthecommunicationflowthatmaycauseabroadcaststormhasalsoincreasedalot.Allinall,theriskofbroadcaststormsintoday'sswitchedLANsisgreatlyexaggerated.IfthemoderateattentionisshiftedtohowtobetterconfiguretheswitchedLAN,thereisnoreasonwhyalargeLANwiththousandsofnodescannotbebuilt,anditstillhastheadvantagesofgoodcostperformanceandscalability.
(4)Transmissionofinformationpacketsbetweensubnets.AlargenumberofappliednetworkprotocolssuchasIP,IPX,andNetBIOSprovideanetworklayeraddressingstructureindependentoftheunderlyingLANtransmission.BothIPandIPXareaddressableprotocols.Inotherwords,theyimplementahierarchicaladdressingscheme,anduseTathagatatoidentifyallnetworkhosts.NetBIOSisanon-addressableprotocol,becausethenetworkhostsimplyidentifiesitwithanamewithoutahierarchicalstructure.Theaddressingstructureofthenetworkprotocolisofgreatsignificancetothedesignofswitchedlocalareanetworks.Becauseofthehierarchicalnatureofnetworkaddresses,networkhostsneedtobedividedintomanygroups,andthehostsineachgrouphavethesamenetworkidentificationnumber.Theonlywayforahostinagrouptocommunicatewithahostinanothergroupistosendthepackettotherouter,andtherouterwillforwardit.
Document1
Store-and-forwardmethod
Themostwidelyusedmethodinthecomputernetworkfield,itputsthedatapacketsoftheinputportfirstStoreit,andthenperformCRCcheck.Afterprocessingtheerrorpacket,thedestinationaddressofthedatapacketistakenout,andthepacketisconvertedintoanoutputportthroughalookuptableandsentout.Becauseofthis,thestore-and-forwardmethodhasalargedelayindataprocessing.Thisisitsshortcoming.ItcanperformerrordetectionondatapacketsenteringtheLANswitch.Itisespeciallyimportantthatitcansupportdifferentspeedsofinputandoutputports.Theconversionbetweenhigh-speedportsandlow-speedportsismaintained.
Layer2andLayer3switchinganditsandrouterThecompetitionofthescheme
TheLANswitchworksatthesecondlayerofOSI,whichcanbeunderstoodasamulti-portbridge,soitistraditionallycalledthesecondlayerswitching;thelatestswitchingtechnologyhasbeenextendedPartofthefunctionsoftheOSIthirdlayer,theso-calledthirdlayerexchange,thethirdlayerexchangecandirectlyusethedynamicallyestablishedMACaddresstocommunicatewithoutspreadingthebroadcastpacket.Itseemsthatthethirdlayerinformation,suchasIPaddress,canbeunderstood.,ARP,etc.,withroutingfunctionsbasedonIP,IPXandotherprotocolsinmulticastandvirtualnetworks.Thesmoothrealizationofthisfunctionisduetotheadditionofapplication-specificintegratedcircuits(ASIC),andthetraditionalinstructionsprocessedbysoftwarearechangedtoASICs.Theembeddedinstructionsofthechipspeeduptheforwardingandfilteringofpackets,sothatlinearroutingandservicequalityathighspeedsarereliablyguaranteed.IfthereisnoneedtogototheWAN,expensiverouterswithlimitedbandwidtharegenerallynolongerusedinnetworkconstructionsolutions.
VirtualLocalAreaNetworkTechnology
Thedevelopmentofswitchingtechnologyallowsregionallydispersedorganizationstologicallybecomeanewworkinggroup,Andmembersofthesameworkinggroupcanchangetheirphysicaladdresseswithouthavingtoreconfigurenodes,whichistheuseofso-calledvirtuallocalareanetworktechnology(VLAN).TheestablishmentofavirtualnetworkwithaLANswitchmeansthattheoriginallargebroadcastarea(allportsoftheswitch)islogicallydividedintoseveral"sub-broadcastareas".Thebroadcastpacketsinthesub-broadcastareawillonlybetransmittedinthebroadcastarea.OthersThebroadcastareaisnotavailable.VLANeffectivelyseparatesthetrafficthroughswitchingtechnology,soastomakebetteruseofbandwidth,andcandividetheactualLANinfrastructureintomultiplesubnetsfromalogicalpointofview.ItallowseachLANtorundifferentapplicationprotocolsandtopologicalstructures.ReaderswhoareinterestedinthedetailsofthispartcanrefertotheIEEE802.10regulations.
LANSwitchMaintenance
IntroductiontoLANSwitches<>IntroductiontoCatalyst6000Series<>IntroductiontoCatalyst5000Series<>Catalyst6000SeriesandCatalyst5000SeriesBasicmaintenancecommands.
OverviewofLANswitches
LANswitcheshavedevelopedtheHUB-typeLANthatpeopleshareinthetraditionalsensetoawiderspace.TaketheCatalyst5000seriesLANswitchproducedbyCiscoasanexample.Itincludesanintegratedswitchinghardwarestructurethatsupportsswitched10-MbpsEthernetand100-MbpsFastEthernet,andcanpassFastEthernet,FDDI,andswitchedtokenring.Exchangeprocessingpowerwithlayer3.Thistypeofswitchcanprovideswitchingaccesstoworkstations,servers,networksegments,backbonenetworks,orotherusersinthelocalareanetwork.