Specifications

Understanding Multicast
April 16, 2009 Page 3 of 32
IGMPusesthreekeycomponentstocontrolmulticastmembership:
• Source—AserverthatsendsanIPmulticastdatastreamwithaparticularmulticast
destinationIPandMACaddress.AservermaynothavedirectIGMPinvolvement,asitoften
doesnotreceiveamulticaststream,butonlysendsa
multicaststream.
• Querier—Adevicethatperiodicallysendsoutqueriesinsearchofmulticasthosts ona
directlyconnectednetwork.IfmultiplequeriersarepresentontheLAN,thequerierwiththe
lowestIPaddressassumestherole.
• Host—AclientendstationthatsendsoneoftwoIGMP
messagestoaquerier:
–Joinmessage—Indicatesthehostwantstoreceivetransmissionsassociatedtoa
particularmulticastgroup.
–Leavemessage—Indicates thehostwantstostopreceivingthemulticasttransmissions.
Figure 1 IGMP Querier Determining Group Membership
AsshowninFig ure 1,amulticast‐enableddevicecanperiodicallyaskitshostsiftheywantto
receivemulticasttraffic.IfthereismorethanonedeviceontheLANperformingIPmulticasting,
oneofthesedevicesiselectedquerierandassu mestheresponsibilityofqueryingtheLANfor
groupmembers.
BasedonthegroupmembershipinformationlearnedfromIGMP,adevicecandeterminewhich(if
any)multicast
trafficneedstobeforwardedtoeachofitsports.AtLayer3,multicastswitch
devicesusethisinformation,alongwithamulticastroutingprotocol,tosupportIPmulticasting
acrosstheInternet.
IGMPprovidesthefinalstepinIPmulticas tdelivery.Itisonly concernedwithforwarding
multicasttrafficfromthe
localswitchdevicetogroupmembersonadirectlyattachedsubnetwork
orLANsegment.
IGMPneitheraltersnorroutesanyIPmulticastpackets.SinceIGMPisnotconcernedwiththe
deliveryofIPmulticastpacketsacrosssubnetworks,anexternalIPmulticastdeviceisneededifIP
multicastpacketshavetobe
routedacrossdifferentsubnetworks.
IGMP Query
IGMP
Querier
Member of
224.1.1.1
Member of
226.7.8.9
IGMP Membership
Router for 226.7.8.9
IGMP Membership
Router for 224.1.1.1