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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