Specifications

How Do I Implement Multicast?
April 16, 2009 Page 2 of 32
How Do I Implement Multicast?
YoucanimplementtheIGMP,DVMRP,andPIMmulticastprotocolsonEnterasysdevicesusing
simpleCLIcommandsasdesc ribedinthisdocument.Abasicconfigurationprocessinvolvesthe
followingtasks:
1. ConfiguringtheVLANsandIPinterfacesonwhichyouwanttotransmitmulticast.
2. Enablingthemulticastprotocol(s)onconfiguredinterfaces.
ForPIM,
youmustalsoconfigureaunicastroutingprotocol,suchasOSPF.
Understanding Multicast
Asdescribedinthe precedingoverview, multicastallowsasourcetosendasinglecopyofdata
usingasingleIP addressfromawell‐definedrangeforanentiregroupofrecipients(amulticast
group).AsourcesendsdatatoamulticastgroupbysimplysettingthedestinationIPaddress
of
thedatagramtobethemulticastgroupaddress.Sourcesdonotneedtoregisterinanywaybefore
theycanbeginsendingdatatoagroup,anddonotneedtobemembersofthegroupthemselves.
Routersbetweenthesourceandrecipientsusethegroupaddresstoroute
thedata,forwarding
duplicatedatapacketsonlywhenthepathtorecipientsdiverges.
Hoststhatwishtoreceivedatafromthemulticastgroupjointhegroupbysendingamessagetoa
multicastrouteronalocalinterface,usingamulticastgroupmembership discoveryprotocol,such
asIGMP.Formoreinformation,
seeInternetGroupManagementProtocol(IGMP)onpage2.
Multicastrouterscommunicateamongthemselvesusingamulticastroutingprotocol, suchas
DVMRPorPIM‐SM.Theseprotocolscalculateamulticastdistributiontreeofrecipientstoensure
that:
•multicasttrafficreachesallrecipientsthathavejoinedthemulticastgroup
•multicasttrafficdoesnot
reachnetworksthatdonothaveanysuchrecipients(unlessthe
networkisatransitnetworkonthewaytootherrecipients)
•thenumberofidenticalcopiesofthesamedataflowingoverthesamelinkisminimized.
Formoreinformation,seeDistanceVectorMulticastRoutingProtocol(DVMRP)onpage5and

ProtocolIndependentMulticast(PIM)onpage10.
Internet Group Management Protocol(IGMP)
Overview
Groupmembershipmanagementisfundamentaltothemulticastingprocess.Anarbitrarygroup
ofreceiverscanexpressinterestinreceivingaparticularmulticaststream,regardlessofthe
physicalorgeographicalboundariesofitsmembers.
ThepurposeofIPmulticastgroupmanagementistooptimizeaswitchednetwork’sperformance
somulticastpacketswill
onlybeforwardedtothoseportscontainingmulticastgrouphostsor
multicastswitchdevicesinsteadoffloodingtoallportsinthesubnet(VLAN).