Bridge

Principle

Anetworkbridgeconnectstwosimilarnetworksandmanagestheflowofnetworkdata.Itworksatthedatalinklayer,whichcannotonlyextendthedistanceorrangeofthenetwork,butalsoimprovetheperformance,reliabilityandsecurityofthenetwork.Afternetwork1andnetwork2areconnectedthroughthebridge,thebridgereceivesthedatapacketsentbynetwork1,andcheckstheaddressinthedatapacket.Iftheaddressbelongstonetwork1,itwilldiscardit.Onthecontrary,ifitistheaddressofnetwork2,itContinuetosendtothenetwork2.Inthisway,thenetworkbridgecanbeusedtoisolatetheinformation,andthesamenetworknumbercanbedividedintomultiplenetworksegments(belongingtothesamenetworknumber)toisolatethesafenetworksegmenttopreventillegalaccessbyusersinothernetworksegments.Duetothesegmentationofthenetwork,eachnetworksegmentisrelativelyindependent(belongingtothesamenetworknumber),andthefailureofonenetworksegmentwillnotaffecttheoperationofanothernetworksegment.

Thenetworkbridgecanbeaspecialhardwaredevice,oritcanberealizedbythenetworkbridgesoftwareinstalledonthecomputer.Atthistime,multiplenetworkadapters(networkcards)willbeinstalledonthecomputer.

Thefunctionofabridgeissimilartoarepeaterinextendingthenetworkspan,butitcanprovideintelligentconnectionservices,thatis,forwardingandfilteringaccordingtowhichnetworksegmenttheendaddressoftheframeisin.Thebridge'sunderstandingofthenetworksegmentwherethesiteislocatedisrealizedby"self-learning",includingtransparentbridges,conversionbridges,encapsulationbridges,andsourceroutingselectionbridges.ThebridgeisshowninFigure1.

WhenusingabridgetoconnecttwoLANs,thebridgemustfirstcheckthedestinationaddressoftheMACframefromthenetworksegment1.Iftheframeissenttoastationonnetworksegment1,thebridgewillnotforwardtheframetonetworksegment2,butfilteritout;iftheframeissenttoastationonnetworksegment2,thebridgeThenitisforwardedtonetworksegment2,whichshowsthatifthereareapairofusersonLAN1andLAN2communicatingsimultaneouslyonthisnetworksegment,itisobviouslyachievable.Becausethebridgeplaysaroleinisolation.Itcanbeseenthatthebridgehastheeffectofincreasingnetworkbandwidthundercertainconditions.

Comparedwithrepeaters,thestoreandforwardfunctionofbridgeshasadvantagesanddisadvantages.Theadvantagesare:

Theuseofbridgesforinterconnectionovercomesphysicallimitations,whichmeansThetotalnumberofdatastationsandnetworksegmentsthatmakeupaLANcanbeeasilyexpanded.

ThestorageandforwardingfunctionofthebridgecanbeadaptedtoconnecttwoLANsusingdifferentMACprotocols,thusformingamixednetworkenvironmentwheredifferentLANsaremixedtogether.

TherelayfunctionofthebridgeonlydependsontheaddressoftheMACframe,soitiscompletelytransparenttothehigher-levelprotocols.

ThebridgedividesalargerLANintosegments,whichhelpstoimprovereliability,availability,andsecurity.

Themaindisadvantageofbridgesisthatsincebridgesreceiveframesandbufferthembeforeforwarding,theyintroducemoredelayscomparedtorepeaters.Becausethebridgedoesnotprovideflowcontrolfunctions,itmaybeoverloadedwhenthetrafficislarge,resultinginframeloss.

Theadvantagesofnetworkbridgesoutweightheirdisadvantages,whichisthereasonwhytheyarewidelyused.

Thebridgeworksatthedatalinklayer.ItconnectstwoLANsandforwardsframesaccordingtotheMACaddress.Itcanberegardedasa"low-levelrouter"(therouterworksatthenetworklayer,andaccordingtothenetworkaddresssuchasIPaddressforforwarding).

TheremotebridgeconnectstworemoteLANsthroughausuallyslowlink(suchasatelephoneline).Forthelocalbridge,performanceismoreimportant.Thedistancetobeabletooperatenormallyismoreimportant.

Prosandcons

Advantagesofbridges

1.Filtertraffic.ThenetworkbridgecanmaketheamountofinformationbetweentheworkstationsonanetworksegmentoftheLANbeconfinedwithinthescopeofthisnetworksegment,andwillnotslipthroughthebridgetoothernetworksegments.

2.ExpandthephysicalrangeandincreasethemaximumnumberofworkstationsontheentireLAN.

3.Differentphysicallayerscanbeusedanddifferentlocalareanetworkscanbeinterconnected.

4.Improvedreliability.IfthelargerLANisdividedintoseveralsmallerLANs,andtheamountofinformationwithineachsmallLANissignificantlyhigherthantheamountofinformationbetweennetworks,thentheperformanceoftheentireinterconnectionnetworkwillbecomebetter.

Disadvantagesofthebridge

1.Sincethebridgemustfirststoreandlookupthestationtableforthereceivedframe,andthenforwardit,thisincreasesthedelay.

2.ThereisnoflowcontrolfunctionintheMACsublayer.Whentheloadonthenetworkisheavy,overflowmayoccurduetoinsufficientstoragespaceofthebridgebuffer,resultinginframeloss.

3.WhennetworksegmentswithdifferentMACsublayersarebridgedtogether,thebridgemustmodifythecontentofcertainfieldsoftheframebeforeforwardingaframetosuittherequirementsofanotherMACsublayer.Increasethedelay.

4.Thenetworkbridgeisonlysuitableforlocalareanetworkswithasmallnumberofusers(nomorethanafewhundred)andasmallamountofinformation.Otherwise,alargebroadcaststormmaysometimesoccur.

Features

Thedatalinklayerinterconnectiondeviceisabridge.Inthenetworkinterconnection,itplaystheroleofdatareceiving,addressfilteringanddataforwardingtoachieveDataexchangebetweenmultiplenetworksystems.

Basicfeaturesofnetworkbridge

1.ThebridgerealizestheLANinterconnectiononthedatalinklayer;

2.Thebridgecaninterconnecttwonetworkswithdifferenttransmissionmediaanddifferenttransmissionrates

3.Thebridgerealizesthecommunicationbetweeninterconnectednetworksbyreceiving,storing,addressfilteringandforwarding;

4.Thenetworksthatneedtobeinterconnectedbythebridgeusethesameprotocolabovethedatalinklayer

5.Thebridgecanseparatethetrafficbetweenthetwonetworks,whichhelpstoimprovetheperformanceandsecurityoftheinterconnectednetwork.

Compatibility

Somepeoplemaynaivelythinkthatthebridgefromone802LANtoanother802LANisverysimple,butinfactitisnotthecase.Amongtheninecombinationsof802.xto802.y,eachhasitsownspecialproblemstobesolved.Beforediscussingthesespecialproblems,let'stakealookatthegeneralproblemsfacedbythesebridges.

Firstofall,variouslocalareanetworksusedifferentframeformats.Thisincompatibilityisnotcausedbytechnicalreasons,butonlybecauseofthecompaniesthatsupportthethreestandards(Xerox,GM,andIBM),noneofthemarewillingtochangethestandardstheysupport.Theresultis:Re-formattingofduplicatedframesbetweendifferentLANsrequiresCPUtime,recalculationofchecksums,andundetectableerrorscausedbybridgestorageerrors.

ThesecondproblemisthatinterconnectedLANsdonothavetooperateatthesamedatatransferrate.WhenafastLANsendsalongseriesofconsecutiveframestoaslowLAN,thespeedofthebridgeprocessingtheframesisslowerthantheframeentryspeed.Bridgesmustusebufferstostoreframesthataretoolatetoprocess,andtheymustalsobewareofrunningoutofmemory.Evenifitisa10Mb/s802.4to10Mb/s802.3bridge,thereissuchaproblemtosomeextent.Becausepartofthe802.3bandwidthisconsumedbyconflicts.802.3isactuallynotreally10Mb/s,and802.4is(almost)10Mb/s.

Asubtleandimportantissuerelatedtothebridgebottleneckproblemisthetimervalueofeachlayeraboveit.Supposethenetworklayeronthe802.4LANwantstosendaverylongmessage(framesequence).Aftersendingthelastframe,itstartsatimerandwaitsforconfirmation.Ifthismessagemustbetransferredtoaslow802.5networkthroughabridge,thetimermayexpirebeforethelastframeisforwardedtothelow-speedLAN.Thenetworklayermayretransmittheentiremessagethinkingthattheframeislost.Afterseveraltransmissionfailures,thenetworklayerwillabandonthetransmissionandtellthetransmissionlayerthatthedestinationsitehasshutdown.

Third,amongalltheproblems,themostseriousproblemisthatthethree802LANshavedifferentmaximumframelengths.For802.3,themaximumframelengthdependsontheconfigurationparameters,butthemaximumpayloadforastandard10M/bssystemis1500bytes.Themaximumframelengthof802.4isfixedat8191bytes.802.5hasnoupperlimit,aslongasthetransmissiontimeofthesitedoesnotexceedthetokenholdingtime.Ifthetokentimedefaultsto10ms,themaximumframelengthis5000bytes.Anobviousquestionarises:WhathappenswhenalongframemustbeforwardedtoaLANthatcannotreceivethelongframe?Inthislayer,theframeisnotconsideredtobedividedintosmallsegments.Allprotocolsassumethattheframeeitherarrivesordoesnotarrive,andthereisnoprovisiontoreassemblesmallerunitsintoframes.Thisisnottosaythatsuchanagreementcannotbedesigned,andthatsuchanagreementcanbedesignedandalreadyexists,but802doesnotprovidethisfunction.Thisproblembasicallycannotbesolved,andframesthataretoolongtobeforwardedmustbediscarded.That'stheleveloftransparency.

Difference

Thebridgedetermineshowtotransmitframesthroughlogicaljudgment.ThislogicisbasedontheEthernetprotocolandcomplieswithOSI'ssecondlayerspecification.Therefore,thebridgecanberegardedasasecond-tierdevice.Usedtodecidewhentoforwardaframe.Editthebridgeasfollows:

1.Checkthereceivedsignal,explainthemeaningof0and1,andfindoutthedestinationMACaddressintheframe.

2.IftheframewiththeMACaddressofthedestinationcanreachthedestinationthroughadifferentinterfaceonthebridge(nottheinterfacewheretheframereachesthebridge),theframeistransmittedbyregeneratingthesignal.(Thisprocessiscalledforwarding.)

3.Iftheinterfaceonwhichtheframearrivesistheportreachablebythedestinationaddress,theframeisdiscarded.(Thisprocessiscalledfiltering.)

Classification

Transparentbridge

Thefirsttypeof802bridgeisatransparentbridgeorspanningtreenetworkBridge(spanningtreebridge).Theprimaryconcernofthosewhosupportthisdesigniscompletetransparency.Accordingtotheirpointofview,afteraunitwithmultipleLANsboughtbacktheIEEEstandardbridge,theyonlyneedtoplugtheconnectionplugintothebridge,andeverythingwillbefine.Noneedtomodifyhardwareandsoftware,noneedtosetaddressswitches,noneedtoloadroutingtablesorparameters.Inshort,donothing,justpluginthecableandit'sdone.TheoperationoftheexistingLANiscompletelyunaffectedbythenetworkbridge.Thisisincredible,andtheysucceededintheend.

Thetransparentbridgeworksinapromiscuousmanner.ItreceiveseveryframetransmittedbyalltheLANsconnectedtoit.Whenaframearrives,thebridgemustdecidewhethertodiscardorforwardit.Ifyouwanttoforward,youmustdecidewhichLANtosendto.Thisneedstobedeterminedbyqueryingthedestinationaddressinalargehashtableinthebridge.Thetablelistseachpossibledestinationandwhichoutputline(LAN)itbelongsto.Atthebeginningoftheinsertionofthebridge,allthehashtablesareempty.Becausethebridgedoesnotknowthelocationofanydestination,afloodingalgorithmisused:eachincomingframewithanunknowndestinationisoutputtoallLANsconnectedtothebridge(excepttheLANthatsendstheframe).Overtime,thebridgewilllearnthelocationofeachdestination.Oncethedestinationlocationisknown,theframessentthereareonlyplacedontheappropriateLAN,andarenolongerdistributed.

Thealgorithmadoptedbythetransparentbridgeisbackwardlearning.Thebridgeworksinapromiscuousmanner,soitcanseetheframestransmittedonanyoftheconnectedLANs.CheckthesourceaddresstoknowwhichmachinecanbeaccessedonwhichLAN,soaddanitemtothehashtable.

Whencomputersandbridgesarepoweredon,poweredoff,ormigrated,thetopologyofthenetworkwillchangeaccordingly.Inordertodealwithdynamictopologyproblems,wheneverahashtableentryisadded,thearrivaltimeoftheframeisindicatedintheentry.Wheneveraframewithadestinationalreadyinthetablearrives,thisitemwillbeupdatedwiththecurrenttime.Inthisway,youcanknowthearrivaltimeofthelastframeofthemachinefromthetimeofeachiteminthetable.Aprocessinthebridgeperiodicallyscansthehashtableandclearsallentriesthatareseveralminutesearlierthanthecurrenttime.Therefore,ifacomputerisremovedfromtheLANandreconnectedtotheLANelsewhere,itcanresumenormalworkwithinafewminuteswithoutmanualintervention.Thisalgorithmalsomeansthatifthemachineisinactiveforafewminutes,theframesenttoitwillhavetobedistributeduntilitsendsoutaframebyitself.

TherouteselectionprocessofthearrivingframedependsonthesendingLAN(sourceLAN)andthedestinationLAN(destinationLAN),asshownbelow:

(1)IfthesourceLANIfitisthesameasthedestinationLAN,theframeisdiscarded.

(2)IfthesourceLANandthedestinationLANaredifferent,theframeisforwarded.

(3)IfthedestinationLANisunknown,thenspread.

Inordertoimprovereliability,somepeoplehavesetuptwoormoreparallelbridgesbetweenLANs.However,thisconfigurationhascausedotherproblemsbecauseloopsaregeneratedinthetopology.Initiateaninfiniteloop.Thesolutionisthespanningtreealgorithmdescribedbelow.

ThesolutiontotheinfiniteloopproblemmentionedaboveistoletthebridgescommunicatewitheachotherandcovertheactualtopologywithaspanningtreereachingeachLAN.Usingspanningtree,youcanensurethatthereisonlyonepathbetweenanytwoLANs.Oncethebridgehasagreedonaspanningtree,alltransmissionsbetweenLANsfollowthisspanningtree.Sincethereisonlyauniquepathfromeachsourcetoeachdestination,therecanbenomorecycles.

Inordertobuildaspanningtree,abridgemustfirstbeselectedastherootofthespanningtree.Themethodofimplementationisthateachbridgebroadcastsitsserialnumber(theserialnumberissetbythemanufacturerandisguaranteedtobeuniqueintheworld),andthebridgewiththesmallestserialnumberisselectedastheroot.Next,constructaspanningtreebasedontheshortestpathfromtheroottoeachbridge.IfabridgeorLANfails,recalculate.

ThebridgescommunicatewitheachotherthroughBPDU(BridgeProtocolDataUnit).Beforethebridgemakesadecisiontoconfigureitself,eachbridgeandeachportneedsthefollowingconfigurationdata:

NetworkBridge:BridgeID(uniqueidentification)

Port:PortID(uniqueidentification)

Portrelativepriority

Thecostofeachport(highbandwidth=Lowcost)

Afterconfiguringeachbridge,thebridgewillautomaticallydeterminethespanningtreeaccordingtotheconfigurationparameters.Thisprocesshasthreestages:

(1)Selecttherootbridge

ThebridgewiththesmallestbridgeIDisselectedastherootbridge.ThebridgeIDshouldbeunique,butiftwobridgeshavethesameminimumID,thebridgewiththesmallerMACaddressisselectedastheroot.

(2)Selecttherootportonallotherbridges

Eachbridgeexcepttherootbridgeneedstoselectarootport,whichshouldbethemostsuitableforcommunicatingwiththerootbridgePort.Bycalculatingthecostofeachporttotherootbridge,takethesmallestoneastherootport.

(3)Selectthe"designatedbridge"and"designatedport"ofeachLAN

IfthereisonlyonebridgeconnectedtoaLAN,itmustbetheLANIfthereismorethanonedesignatedbridge,theonewiththeleastcosttotherootbridgeisselectedasthedesignatedbridgefortheLAN.ThedesignatedportconnectsthedesignatedbridgeandthecorrespondingLAN(iftherearemorethanonesuchports,thelowerpriorityisselected).

Aportmustbeoneofthefollowing:

(1)Rootport

(2)AdesignatedportofaLAN

(3)Blockedport

Whenabridgeispoweredon,itassumesthatitistherootbridgeandsendsoutaCBPDU(ConfigurationBridgeProtocolDataUnit)toinformitoftherootbridgeIDitthinks.WhenabridgereceivesaCBPDUwhoserootbridgeIDislessthanitsknownID,itwillupdateitstable,andiftheframearrivesfromtherootport(upload),itwillbedistributedtoalldesignatedports(download).WhenabridgereceivesaCBPDUwitharootbridgeIDgreaterthanitsknownID,theinformationisdiscarded.Iftheframearrivesfromthedesignatedport,aframeissentbacktoinformthelowerIDoftherealrootbridge.

Whenthenetworkisreconfiguredintentionallyorduetoalinefailure,theaboveprocesswillberepeatedtogenerateanewspanningtree.

Source-routedbridge

Theadvantageoftransparentbridgeisthatitiseasytoinstall,justpluginthecableandyou'redone.Butontheotherhand,suchbridgesdonotmakethebestuseofbandwidthbecausetheyonlyuseasubsetofthetopology(spanningtree).Therelativeimportanceofthesetwo(orother)factorsledtoasplitwithinthe802committee.ThosewhosupportCSMA/CDandtokenbuschosetransparentbridges,whileproponentsoftokenringpreferredabridgecalledsourcerouting(encouragedbyIBM).

Thecoreideaof​​sourceroutingistoassumethatthesenderofeachframeknowswhetherthereceiverisonthesameLAN.WhensendingaframetoanotherLAN,thesourcemachinesetsthehighbitofthedestinationaddressto1asamark.Inaddition,itaddstheactualpaththattheframeshouldtakeintheframeheader.

Thesourcerouteselectionbridgeonlycaresaboutthoseframeswhosedestinationaddressishighin1,andwhenitseessuchaframe,itscanstherouteintheframeheaderandlooksforthenumberoftheLANthatsenttheframe.IftheLANnumberthatsenttheframeisfollowedbythenumberofthebridge,thentheframewillbeforwardedtotheLANbehinditselfintheroutingtable.IftheLANnumberisnotfollowedbythelocalbridge,theframewillnotbeforwarded.Therearethreepossiblespecificimplementationsofthisalgorithm:software,hardware,andhybrid.Thepriceandperformanceofthesethreespecificimplementationsaredifferent.Thefirsttypehasnointerfacehardwareoverhead,butrequiresafastCPUtoprocessallincomingframes.ThelastimplementationrequiresaspecialVLSIchip,whichsharesalotoftheworkofthebridge.Therefore,thebridgecanuseaslowerCPUorcanconnectmoreLANs.

ThepremiseofsourceroutingselectionisthateverymachineintheInternetknowsthebestpathforallothermachines.Howtogettheseroutesisanimportantpartofthesourceroutingalgorithm.Thebasicideaof​​obtainingtheroutingalgorithmis:ifthelocationofthedestinationaddressisnotknown,thesourcemachinewillpublishabroadcastframeandaskwhereitis.EachbridgeforwardsthediscoveryframesothattheframecanreacheveryLANintheInternet.Whenthereplycomesback,thebridgespassingbywillrecordtheirownidentificationinthereplyframe,sothesenderofthebroadcastframecangettheexactrouteandchoosethebestroutefromit.

Althoughthisalgorithmcanfindthebestroute(itfindsallroutes),italsofacestheproblemofframeexplosion.Asimilarsituationoccurswithtransparentbridges,butitisnotasserious.Itsspreadiscarriedoutaccordingtothespanningtree,sothetotalnumberofframestransmittedisalinearfunctionofthenetworksize,unlikesourceroutingwhichisanexponentialfunction.Oncethehostfindsaroutetoacertaindestination,itstoresitinthehigh-speedbufferandnolongerneedstolookup.Althoughthismethodgreatlysuppressesframeexplosion,itaddstransactionalburdentoallhosts,andtheentirealgorithmisdefinitelyopaque.

TransparentbridgesaregenerallyusedtoconnectEthernetsegments,whilesourceroutingbridgesaregenerallyusedtoconnecttokenringnetworksegments.

Difference

Comparisonbetweenbridgeandrouter

Thebridgedoesnotknowtheinformationofthehigher-levelprotocolintheforwardedframe,whichallowsittousethesameIthandlesIP,IPXandotherprotocols,anditalsoprovidesthefunctionofsegmentingnetworkswithoutroutingprotocols(suchasNetBEUI).

Becauseroutersprocessdataatthenetworklayer,itiseasierforthemtointerconnectdifferentdatalinklayers,suchastokenringnetworksegmentsandEthernetsegments.Bridgesaregenerallymoredifficulttocontrolthanrouters.ProtocolssuchasIPhavecomplexroutingprotocolsthatmakeiteasyfornetworkmanagerstomanagerouting;protocolssuchasIPalsoprovidemoreinformationonhowtosegmentthenetwork(evenifitsaddressalsoprovidessuchinformation).ThebridgeonlyusestheMACaddressandphysicaltopologytowork.Therefore,bridgesaregenerallysuitableforsmallandsimplenetworks.

Howtooperate

NetworkbridgesaresometimesusedtoconnecttwoormoreremoteLANs.Forexample,ifacompanyisdistributedinmultiplecities,thecompanyhasalocalLANineachcity.TheidealsituationisthatallLANsareconnected,andtheentiresystemislikealargeLAN.

Thisgoalcanbeachievedbythefollowingmethod:setupanetworkbridgeineachLAN,anduseapoint-to-pointconnection(suchasrentingatelephonelinefromatelephonecompany)toconnectthemtwobytwostandup.Avarietyofdifferentprotocolscanbeusedforpoint-to-pointconnections.Oneofthemethodsistoselectacertainstandardpoint-to-pointdatalinkprotocolandaddthecompleteMACframetothepayload.IfallLANsarethesame,thisapproachworksbest.ItsonlyproblemisthattheframesmustbesenttothecorrectLAN.AnothermethodistoremovetheMACheaderandtailinthesourcebridge,andaddtheremainingparttothepoint-to-pointprotocolpayload,andthengeneratenewheadersandtailsinthedestinationbridge.Itsdisadvantageisthatthechecksumthatreachesthedestinationhostisnotthechecksumcalculatedbythesourcehost,sotheerrorcausedbyadamagedbitinthebridgememorymaynotbedetected.

Relatedcases

Manyorganizationshavemultiplelocalareanetworksandhopetoconnectthem.OneunithasmultipleLANsforthefollowingreasons:

Firstofall,manyuniversitydepartmentsorcompanydepartmentshavetheirownLANs,whicharemainlyusedtoconnecttheirownpersonalcomputers,workstations,andservers.Duetothedifferentnatureoftheworkofeachdepartment(ordepartment),differentlocalareanetworksareselected.Soonerorlater,thesedepartments(ordepartments)needtocommunicatewitheachother,andthereforeneednetworkbridges.

Secondly,aunitisgeographicallydispersedandfarapart.Insteadofinstallingacoaxialcablenetworkspreadoveralllocations,itisbettertoestablishalocalareanetworkateachlocationandusebridgesandinfraredlinks.Connected,sothecostmaybelower.

Third,itmaybenecessarytodividealogicallysingleLANintomultipleLANstoadjusttheload.Forexample,multiplelocalareanetworksconnectedbybridgesareused.Eachlocalareanetworkhasagroupofworkstationsanditsownfileserver.Therefore,mostofthecommunicationislimitedtoasinglelocalareanetwork,whichreducestheburdenonthebackbonenetwork.

Fourth,insomecases,asingleLANisnoproblemfromtheloadpointofview,butthephysicaldistancebetweenthefarthestmachinesistoofar(forexample,morethanthe2.5kmspecifiedby802.3).Evenifthecablelayingisnotaproblem,thenetworkwillstillnotworknormallyduetothelonground-tripdelay.TheonlywayistosegmenttheLANandplacebridgesbetweenthesegments.Byusingabridge,youcanincreasethetotalphysicaldistanceofwork.

Fifth,reliabilityissues.Inasinglelocalareanetwork,adefectivenodecontinuouslyoutputtinguselessinformationflowwillseriouslydamagetheoperationofthelocalareanetwork.Thebridgecanbesetinthekeypartofthelocalareanetwork,justlikethefiredoorinthebuilding,topreventdamagetotheentiresystemduetothemalfunctionofasinglenode.

Sixth,networkbridgesarehelpfulforsecurityandconfidentiality.MostLANinterfaceshaveapromiscuousmode,inwhichthecomputerreceivesallframes,includingthosethatarenotaddressedtoit.Ifbridgesaresetupinmultipleplacesinthenetworkandimportantinformationthatdoesnotneedtobeforwardediscarefullyintercepted,thenthenetworkcanbeseparatedtopreventinformationfrombeingstolen.