路由器在日常应用中常见问题和解答Commonproblemsandsolutionsfor

路由器在日常应用中常见问题和解答（Commonproblemsandsolutionsforroutersineverydayapplications)Whentouserouteredistribution?Routingredistributionisusuallyconfiguredonroutersthatareresponsibleforlearningfromanautonomoussystemandthenbroadcastingtoanotherautonomoussystem.IfyouareusingI,G,R,P,orEIGRP,routingredistributionisusuallyautomatic.Whatisthemanagementdistance?Managementdistancereferstotheroutingcredibilityofaroutingprotocol.Eachroutingprotocolassignsatrustlevelinorderofreliabilityfromhightolow,andthistrustleveliscalledmanagementdistance.Forroutinginformationoftwodifferentroutingprotocolstoadestination,therouterfirstdecideswhichprotocoltotrustaccordingtothemanagementdistance.HowdoIconfigureredistribution?Beforeroutingredistribution,youhavetostartwith:decidewheretoaddanewprotocol.determinetheautonomoussystemboundaryrouter(ASBR).decidewhichagreementisatthecoreandwhichisattheborder.determinethedirectionofroutingredistribution.Youcanusethefollowingcommandtoredistributeroutingupdates(thisexampleisforOSPF):Router(config-router)#redistribute,[process-id],[metric,metric,protocol-value][metric-typetype-value][subnets]Inthiscommand:Protocol:thesourceroutingprotocolthatspecifiestherouteredistributionoftherouter.Themainvaluesare:BGP,EqP,IGRP,Isis,OSPF,static,[IP],connected,andrip.Process-id:indicatestheprocessID.ofOSPFMetric:anoptionalparameterthatspecifiesthemetricoftheredistributionroute.Thedefaultmeasureis0.Whyisitimportanttoidentifyadjacentrouters?Determiningadjacentroutersinasmallnetworkisnotamajorconcern.Becausewhenarouterfails,otherrouterscanconvergewithinanacceptabletime.Butinlargenetworks,afaultroutercanbefoundwithaverylargelatency.KnowingthatadjacentrouterscanspeedupconvergenceisbecauseroutersaremorelikelytoknowthefaultroutersbecausetheintervalsofHellomessagesareshorterthanthetimeintervalsforrouterstoexchangemessages.Whenusingadistancevectorroutingprotocol,therouterdoesnotsendrouteupdateinformationtotheadjacentrouter,anditcanbefoundthattheadjacentrouterisunreachable.Thistimeisusually10~90seconds.RoutersthatuselinkstateroutingprotocolsdonotreceiveHellomessagesandfindthatadjacentroutersareunreachable.Thisintervalistypically10seconds.Howdodistancevectorroutingprotocolsandlinkstateroutingprotocolsdiscoveradjacentrouters?Arouterusingadistancevectorroutingprotocolcreatesaroutingtablethatincludesthenetworkdirectlyconnectedtoit,andsendstheroutingtabletoarouterthatisdirectlyconnectedtoit.Theneighborroutermergesthereceivedroutingtableintoitsownroutingtable,andalsosendsitsroutingtabletoitsneighborrouter.RoutersusinglinkstateroutingprotocolscreatealistoflinkedStates,includingalistofdestinationstationsthroughoutthenetwork.Intheupdatemessage,eachroutersendsitsentirelist.Whentheneighborrouterreceivestheupdatemessage,itcopiesthecontentsandsendstheinformationtoitsneighbor.Thereisnoneedtorecalculatewhenforwardingthecontentsoftheroutingtable.NoticetheuseofIGRPandEIGRProuterstobroadcasthellopacketstofindadjacentstationsandtoexchangeroutingupdateslikeOSPF.EIGRPforeachnetworklayerprotocolmaintainsaneighbortable,itincludestheadjacentstationaddress,waitinthequeueforthenumberoftransmittedmessages,fromneighbortoneighborstationstationreceivingorsendingmessagetotheaveragetime,andnotbeforedeterminingthelinkdisconnectedfromtheadjacentstationreceivedanymessagetime.Whatisanautonomoussystem?Anautonomoussystemisarouterandnetworkgroupthatisunderthecontrolofamanagementauthority.It canbearouterthatconnectsdirectlytoanLANandconnectstoInternet;itcanbeaLANthatisinterconnectedbyanenterprisebackbone.Allroutersinanautonomoussystemmustbeconnectedtoeachother,runningthesameroutingprotocol,andallocatingthesameautonomoussystemnumber.Linksbetweenautonomoussystemsuseexternalroutingprotocols,suchasB,G,P.WhatisBGP?BGP(BorderGatewayProtocol)isaroutingprotocolthatdynamicallyexchangesroutinginformationbetweenautonomoussystems.AclassicdefinitionofanautonomoussystemisasetofroutersunderthecontrolofanadministrationthatusesIGPandcommonmetricstoforwardmessagestootherautonomoussystems.TheuseoftheautonomoussystemintheBGPthistermistoemphasizethefactthatanautonomoussystemofmanagementistoprovideaunifiedinternalroutingschemeforotherautonomoussystems,itprovidesaconsistentdescriptionforthoseyoucanreachitthroughthenetwork.BGPsupportedsessiontypesBGPsessionsbetweenadjacentroutersarebuiltontopoftheTCPprotocol.TheTCPprotocolprovidesareliabletransportmechanismthatsupportstwotypesofsessions:ExternalBGP(EBGP):asessionbetweenroutersbelongingtotwodifferentautonomoussystems.Theseroutersareadjacentandsharethesamemediumandsubnet.InternalBGP(IBGP):asessionbetweenrouterswithinanautonomoussystem.Itisusedtocoordinateandsynchronizetheroutingprocesswithinanautonomoussystem.BGProuterscanbelocatedanywhere,eveninthemiddleofanautonomoussystem,andcanbeseparatedbyseveralrouters.NotethatthecontentsoftheinitialdatastreamaretheentireBGProutingtable.However,whentheroutingtablechanges,theroutertransmitsonlythechangedpart.TheBGPdoesnotneedtoperiodicallyupdatetheentireroutingtable.Thus,duringtheconnectionestablishmentperiod,aBGPsendermustholdtheentireBGProutingtablethatiscommontoallexistingpeerrouters.TheBGProuterperiodicallysendsKeepAlivemessagestoverifythattheconnectionisactive.Whenanerrororaparticularcaseoccurs,theroutersendstheNotificationmessage.Whenanerroroccursinaconnection,anotificationmessageisgeneratedandtheconnectionisbroken."-fromRFC11654andBGP*.DoesBGPallowrouteredistribution?Permit.BecauseBGPisusedprimarilyforroutingbetweenautonomoussystems,itmustsupporttheintegrationofRIP,OSPF,andIGRProutingtablesinordertotransfertheirroutingtablestoanautonomoussystem.BGPisanexternalroutingprotocol,soits*operationdiffersfromthatofaninternalroutingprotocol.InBGP,aroutecanonlybecreatedintheBGProutingtablewiththeNETWORKcommandonlywhenaroutealreadyexistsintheIProutingtable.HowdoIdisplayallBGProutesinthedatabase?TodisplayalloftheBGProutesinthedatabase,simplyenterattheEXECcommandline:ShowipBGPpathsTheoutputofthiscommandmaybe:AddressHashRefcountMetricPath0x297A9C020IWhatishorizontalsegmentation?Horizontalsegmentationisatechniquetoavoidtheemergenceofroutingloopsandtospeeduproutingaggregation.Becausetheroutermayreceiveitssendroutinginformation,andthisinformationisuseless,donotnoticeanyreverselevelsegmentationfromtheterminalreceiveroutingupdates,butonlythosewhowillnotnoticeclearbecausethecountingtoinfinityrouting.Howistheroutingringgenerated?Becauseoftheexistenceofroutingconvergencetimeofthenetwork,newroutingoralteredroutinginroutingtablescannotbestabilizedinthewholenetworkveryquickly,sothatthereareinconsistentrouting,andthentheroutingringwillbegenerated.Whatisameasure?Measurerepresentsdistance.Theyareusedtodeterminetheoptimalroutewhensearchingforroutes.Eachroutingalgorithmproducesaroutingtablethatgeneratesavalue(Metric)foreachpaththroughthenetwork,andtheminimumvalueindicatestheoptimalpath.Thecalculationofmetricscanconsideronlyonecharacteristicofthepath,butmorecomplexmeasuresaregeneratedbycombiningmultiplecharacteristicsofthepath.Somecommonlyusedmetricsare:Hopcount:thenumberofportsthatamessagewillpassthroughtherouter.Ticks:datalinkdelay(about1/18persecond).Price:canbeanarbitraryvalue,isbasedonbandwidth,cost,orothernetworkmanagersdefinedbythecalculation.Bandwidth:thecapacityofadatalink.Delay:thelengthoftimeamessagetravelsfromitssourcetoitsdestination.Load:thesizeofapartofanetworkresourceorlinkthathasbeenused.Reliability:theratiooftheerrorbitsofanetworklink.Maximumtransferunit(MTU):maximumacceptablemessagelength(inbytes)ofalllinksacceptedonapath.WhattypeofroutingmetricdoesIGRPuse?Whatisthismetricmadeof?IGRPusesmultipleroutingmetrics.Itincludesthefollowingsections:Bandwidth:theminimumbandwidthvaluebetweensourceanddestination.Delay:theaccumulationofinterfacedelaysinapath.Reliability:theworstpossiblereliabilitybetweensourceanddestination,basedonlinkmaintenance.Load:thelinkbetweensourceanddestination;theworstcaseloadisexpressedinbitspersecond.MTU:theminimumM,T,Uvalueinthepath.Canmeasurementsbemodifiedoradjusted?Addapositiveoffset.Thecompletestructureofthiscommandisasfollows:youcanusetheOFFSET-LISTROUTERchildcommandAddsapositiveoffsettothenetworkinputandoutputmetricsinthetable.Offset-list,{in,out},offset,[access-list],no,offset-list,{in,,out},offset,[access-list]IfthevalueofparameterLISTis0,thentheOFFSETparameterisaddedtoallofthemeasurements.IfthevalueofOFFSETis0,thenthereisnofunction.ForIGRP,theoffsetvalueisonlyappliedtothedelay.ThissubcommandalsoappliestoRIPandhelloroutingprotocols.YoucanclearthisoffsetusingtheNOOFFSET-LISTcommandwiththeappropriatearguments.Inthefollowingexample,arouterusingIGRPaddsoffset10:offset-listout10tothedelayofalloutputmetricsHereisanexampleofaddingthesameoffset