LANChapter13Word文件下载.docx

LANChapter13Word文件下载.docx

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Coverstheprotocoloperations,messageformats,andoptionsinadditiontohowCGMPandIGMPworkinroutedandswitchedenvironments.ThissectionalsoprovidesdetailsonthestepsnecessarytomaketheseprotocolsactiveinyourCatalystsandrouters.

∙IGMPSnooping:

CoversanothermethodofcontrollingmulticastsinaCatalystnetwork.

∙BroadcastSuppression:

ExplainsbroadcastsuppressionmethodsinaCatalystnetwork.

Manycorporationsandindustriesnowrealizethepotentialofmultimediaapplications.Videoandvoiceconferenceapplicationsgeneratemuchofthemulticasttraffictoday.ManyWebsitesoffervideostreamsthattransmitmulticasttraffic.Organizations,therefore,areexperiencinganincreaseinmulticasttrafficfrominternalusersandfromWebdownloads.Ifyoudonotseemuchmulticasttrafficinyoursystemtoday,bepreparedbecauseyousoonwill.Asaresultoftheproliferationofmulticastapplications,networkadministratorshavetoplananddealwithmulticasttrafficloadsinmorefrequencythaninthepast.Manynetworkplansdealefficientlywithunicasttraffic,butneglecttheimpactofmulticastservicesinthenetwork.Unfortunately,thisseverelyimpactsthewell-thought-outunicastnetwork.

Withmulticasttrafficpresentinnetworks,wisdomdictatesthatyoushouldproactivelycontrolthedistributionofmulticastthroughoutyournetwork.Thedefaultbehaviorofaswitchfloodstheframeeverywhereinthebroadcastdomain.Thisisusuallynotnecessary,buthappensbecauseofbridgerules.CiscoimplementstheproprietaryCiscoGroupManagementProtocol（CGMP）tohelpyouadministermulticastinyournetwork.CGMPworksinaCatalystandrouteralongwithInternetGroupMembershipProtocol（IGMP）inaroutertocontrolthefloodingofmulticasttrafficinanetwork.TheCatalyst5000,6000,4000,2948G,2926G,and2926modelssupportmulticastsuppressionthroughCGMP.

Althoughthischapterdoesnotintendtobeadefinitivesourceformulticastissues,abriefoverviewofmulticastusage,addressing,IGMPoperations（bothversion1andversion2）,andCGMPisgermanetotheCatalyst.Thefollowingsections,therefore,providebackgroundinformationonthesetopics.

CGMP/IGMP:

AdvancedTrafficManagement

Multicasttrafficoriginatesatsourcesdesiringtodistributethesameinformationtomultiplerecipients.Whenasourcecreatesmulticasttraffic,itusesspecialLayer2andLayer3addressessothatroutersandbridgesknowhowtodispersetheframe.Bydefault,routersdonotforwardmulticasttrafficunlesstheyaremulticastcapableandhaveamulticastroutingprotocolsuchasDVMRP（distancevectormulticastroutingprotocol）orPIM（protocolindependentmulticast）enabled.DVMRPandPIMareinter-routerprotocols.Therefore,hostsandswitchesdonotparticipateinmessagesfortheseprotocols.

ALANswitchisnotarouter（althougharoutercanbeincorporated,suchastheRSM）.Whathappens,then,tomulticasttrafficinaswitchednetwork?

Bydefault,aswitch（bridge）floodsmulticasttrafficwithinabroadcastdomain.Thisconsumesbandwidthonaccesslinksandtrunklinks.Dependinguponthehost'

sTCP/IPstackimplementationandnetworkinterfacecard（NIC）attributes,themulticastframecancauseaCPUinterrupt.Whydoesaswitchfloodmulticasttraffic?

Aswitchfloodsmulticasttrafficbecauseithasnoentryinthebridgetableforthedestinationaddress.Multicastaddressesneverappearassourceaddresses,thereforethebridge/switchcannotdynamicallylearnmulticastaddresses.Youcanmanuallyconfigureanentrywiththesetcamstaticcommand.

IGMPisamulticastprotocolthatdirectlyaffectshosts.IGMPallowshoststoinformroutersthattheywanttoreceivemulticasttrafficforaspecificmulticastgroupaddress.

CurrentCatalystsdon'

tunderstandIGMPmessages（unlessyouhavetheNetFlowFeatureCard[NFFC]）.IGMPmessagesappeartoaCatalystlikeanyothermulticastframes.CiscodevelopedtheproprietaryCGMPthatenablesrouterstoinformCatalystsabouthostsandtheirinterestinreceivingmulticasttraffic.ThismodifiestheCatalyst'

sdefaultbehavioroffloodingthemulticastframetoallhostsinthebroadcastdomain.Ratherthanfloodingtheframetoallhosts,theCatalystlimitsthefloodingscopetoonlythosehostsinthebroadcastdomainthatregisteredwiththerouterthroughIGMP.Ifahostdoesnotregisterwiththerouter,itdoesnotreceiveacopyofthemulticastframe.Thispreservesaccesslinkbandwidth.

MulticastAddresses

Wheneveranapplicationneedstosenddatatomorethanonestation,butwantstorestrictthedistributiontoonlystationsinterestedinreceivingthetraffic,theapplicationtypicallyusesamulticastdestinationaddress.Multicastaddressestargetasubsetofallstationsinthenetwork.Theothertwotransmissionchoicesforatransmittingdeviceareunicastorbroadcastframes.Ifthesourceusesabroadcastaddress,allstationsinthebroadcastdomainmustprocesstheframe,eveniftheyarenotinterestedintheinformation.Ifthesourcetransmitsunicastframes,itmustsendmultiplecopiesoftheframe,oneaddressedtoeachintendedreceiver.Thisisaveryinefficientuseofnetworkresourcesanddoesnotscalewellasthenumberofreceiversincreases.

Byusingmulticastaddresses,thesourcetransmitsonlyonecopyoftheframeontothewireandroutersdistributethemulticastmessagetotheothersegmentswhereinterestedreceiversreside.MulticastaddressesappearatLayer2andatLayer3.AnetworkadministratorassignsthemulticastLayer3addressforanapplication.TheLayer2multicastaddressisthencalculatedfromtheLayer3multicastaddress.Thisisshowninthesection,"

Layer2MulticastAddresses."

Whenyouconfigureamulticastapplication,theNICaddsthemulticastaddresstoitslistofvalidMACaddresses.Usuallythislistconsistsofthebuilt-inMACaddressplusanyuserconfiguredaddresses.Wheneverthestationreceivesaframewithamatchingmulticastdestinationaddress,thereceiversendstheframetotheCPU.

TherouterexaminesmulticastaddressesatbothLayer2andLayer3,whereasaswitchexaminestheLayer2address.IftheswitchhashardwaresuchasaCatalyst5000SupervisorIIImodulewithanNFFC,theCatalystcanexaminetheLayer3addressesaswell.（Theadvantageofthisisseeninthesection,"

IGMPSnooping:

AdvancedTrafficManagement,"

laterinthechapter）.Otherwise,theCatalystsimplyexaminestheMACaddressintheframe.

Layer3MulticastAddresses

IPmulticastaddressesatLayer3arecharacterizedasclassDaddresses.ThefirstfourbitsofIPclassDaddressesare1110.ThismeansthatIPmulticastaddresseshaveavalidrangefrom224.0.0.0to239.255.255.255.Notethatmulticastaddressesfrom224.0.0.0to224.0.0.255arereserved.Thefollowingthreereservedaddressesarespecificallyinterestingtothisdiscussion:

∙224.0.0.1— 

Allmulticastcapablehostsonthesegment

∙224.0.0.2— 

Allmulticastcapableroutersonthesegment

∙224.0.0.4— 

AllDVMRProutersonthesegment

Usually,anetworkadministratorassignsamulticastaddresstotheapplicationandmustselectanaddressnotusedbyotherapplicationsorprocesses.Theadministratorshouldnotuseamulticastaddressinthereservedrange.

Layer2MulticastAddresses

WhenyouassignaLayer3multicastaddress,aLayer2addressisautomaticallygeneratedfromtheIPaddress.Figure13-1showshowaMACmulticastaddressisderivedfromanIPmulticastaddress.TocalculatetheLayer2address,thehostcopiesthelast23bitsoftheIPaddressintothelast24bitsoftheMACaddress.Thehighorderbitissetto0.

Figure13-1CalculatingaMulticastMACAddress

Thefirst3bytes（24bits）ofthemulticastMACaddressare0x01-00-5E.ThisisareservedOUIvalueindicatingamulticastapplication.

Consideranexample.TheIPaddress224.1.10.10,assignedbyanadministrator,hasalow23-bitvalueof1.10.10.Inhexadecimalformat,thisis0x01-0A-0A.TheMACaddresstakesthelast23ofthe24bitsandplacesthemintotheMACfield.ThecompleteMACaddressinthiscaseis01-00-5E-01-0A-0A.

WhathappensiftheIPmulticastaddressis225.1.10.10?

Asideeffectofthisschemeisaddressambiguity.AlthoughadifferentIPmulticastgroupisidentifiedatLayer3,theLayer2addressisthesameasfor224.1.10.10.Layer2devicescannotdistinguishthetwomulticastgroupsandreceiveframesfrombothmulticastgroups.Theuserapplicationneedstofilterthetwostreamsanddiscardtheunwantedmulticastframes.Anybitvaluecombinationforthe5bitsinFigure13-1identifiedasambiguousgeneratesthesameLayer2MACmulticastaddress.Fivebitsmeansthatthereare25combinations,or32Layer3addressesthatcreatethesameLayer2address.

Tip

Whenassigningmulticastaddresses,besuretorememberthe32:

1ambiguityandtrytoavoidmulticastoverlaps.Thishelpstopreservebandwidthonaccessandtrunklinks.TheendstationdiscardstheunwantedmulticastatLayer3,afteritinterruptstheCPU.

IGMP

IGMPdefinesaprotocolforhoststoregisterwitharoutertoreceivemulticasttrafficforaspecificmulticastgroup.TwoversionsofIGMPexist:

version1,specifiedinRFC1112,andversion2,specifiedinRFC2236.Version2addedsignificantfeaturestoversion1makingitmoreefficientandenablinghoststoexplicitlyleaveamulticastgroup.ThesefeaturesaredescribedinthesectiononIGMPversion2.

IGMPVersion1

Figure13-2showsthe8-octetIGMPversion1framef