理解bgp多出口鉴别器

RouteSelectionUsingAttributesUnderstandingBGPMulti-ExitDiscriminatorsOutlineOverviewSelectingtheProperReturnPathMEDPropagationinaBGPNetworkChangingtheDefaultMEDChangingtheMEDwithRoute-MapsAdvancedMEDConfigurationMonitoringtheMEDTroubleshootingtheMEDSummarySelectingtheProperReturnPathQ: Howcanyoumakesurethatthereturntraffictakestherightpath?Multi-ExitDiscriminatorYoucanusetheMEDtoinfluencepathselectioninneighborautonomoussystems.-可以用MED影响邻居的路径选择AnAScanspecifyitspreferredentrypointusingtheMEDinoutgoingEBGPupdates.-一个AS可以在EBGP的路由更新的时候给邻居指定流量的优先入口TheMEDisnotpropagatedoutsideofareceivingAS.MED值不会传出接收ASThedefaultvalueoftheMEDattributeis0.缺省的MED为0TheMEDiscalledthe“metric”inCiscoIOSsoftware.TheMEDisa“weak”metric.AlowerMEDvaluemeansmorepreferred.－MED越小越优先MEDPropagationinaBGPNetworkChangingtheDefaultMEDdefault-metricnumberrouter(config-router)#TheMEDiscopiedfromtheIGPcostintherouterthatsourcestheroute(throughthenetworkcommandorthroughrouteredistribution).YoucanchangetheMEDvalueforredistributedrouteswiththedefault-metriccommand.ChangingtheMEDwithRoute-Mapsroute-mapnamepermitsequence

matchconditionsetmetricvalue

router(config)#ChangesMEDforroutesmatchedbytheroute-mapentryneighboraddressroute-mapnamein|outrouter(config-router)#Appliesaroute-maptoingupdatesfromaspecifiedneighborortooutgoingupdatestoaspecifiedneighborPer-neighborMEDconfiguredbyusingaroute-mapwithnomatchconditionChangingtheMEDwithRoute-Maps(Cont.)AdvancedMEDConfigurationbgppare-med router(config-router)#Bydefault,theMEDisconsideredonlyduringselectionofroutesfromthesameAS.Withpare-med,theMEDisalsoconsideredforroutescomingfromadifferentAS.bgpbestpathmedmissing-med-worstrouter(config-router)#IftheMEDisnotattachedtoaBGProute,itisinterpretedasvalue0,andthusasthebestmetric.Withthiscommand,amissingMEDisinterpretedasinfinity(worst).AdvancedMEDConfiguration(Cont.)bgpbestpathmed

confedrouter(config-router)#Bydefault,theMEDisconsideredonlyduringselectionofroutesfromthesameAS,whichdoesnotincludeintra-confederationautonomoussystems.Usethiscommandtoallowrouterstocomparepathslearnedfromconfederationpeers.bgpdeterministic-medrouter(config-router)#ThiscommandchangestheBGProuteselectionproceduretoadeterministicbutslowerone.MonitoringtheMEDTheMEDisdisplayedinshowipbgp[prefix]printoutasthe“metric”field.TheMEDafterroute-mapprocessingisdisplayedinBGPupdatedebugging.TheMEDreceivedfromaneighborisdisplayedinshowipbgpneighborreceived-routesprintouts.MonitoringtheMED(Cont.)MonitoringtheMED(Cont.)MEDisdisplayedinshowipbgpprintout.MonitoringtheMED(Cont.)MEDvaluesarealsodisplayedinshowipbgpprefixprintout.SummaryTheMEDisa“weak”parameterintherouteselectionprocess—itisusedonlyifweight,localpreference,ASpath,andorigincodeareequal.Bydefault,theMEDiscomparedonlyforpathsthatwerereceivedfromthesameAS.TheMEDisnotamandatoryattributeandisnormallynotpresentinBGPupdates,exceptwhenarouteroriginatesanetworkthathasanexactmatchintheroutingtable—theMEDisgivenavaluecopiedfromthemetricintheroutingtable.ThereisnoMEDattributethatisattachedtoaroutebydefault,sotosetthedefaultmetricvalue(MED)forBGProutes,usethedefault-metriccommandinrouterconfigurationmode.Youcanusearoute‑maptosetanarbitraryMEDvaluetosentorreceivedroutes.Summary(Cont.)YoucanconfigureadvancedMEDparameterstomodifythedefaultMEDbehaviors.Forexample,thebgppare-medcommandforcestheroutertocomparetheMEDevenifpathscamefromdifferentautonomoussystems,andyoumustusethebgpbestpathmedconfedcommandwhenyouusetheMEDwithinaconfederationtoinfluencetheroute