Specifications

Understanding Multicast
April 16, 2009 Page 5 of 32
EachrouterperformsanIGMPforwardingchecktoseeifthereareanyhoststhatwanttojoin
themulticastgrouponitslocallyattached network.Eachrouterdropsmulticastpacketsuntil
ahostjoinsthegroupusingoneofthefollowingmessages:
– solicitedjoin(sentinresponsetoanIGMP
queryproducedbytherouter’sinterface)
InFigure 2,thistypeofexchangeoccursbetweenRouter1andHost1when:
(3) Router1sendsaquerytopotentialHost1.
(4) Host1respondswithajoinmessage.
(5) Router1forwardsthemulticaststream.
– unsolicitedjoin(sentasarequestwithoutreceivingan
IGMPqueryfirst)
InFigure 2,thistypeofexchangeoccursbetweenRouter2andHost2when:
(6) Host2sendsajoinmessagetoRouter2.
(7) Router2forwardsthemulticaststreamtoHost2.
(8) Whenitnolongerwantstoreceivethestream,Host2candooneofthefollowing:
‐ SendaleavemessagetoRouter2.
‐ TimeouttheIGMPentrybynotrespondingtofurtherqueriesfromRouter2.
Distance Vector Multicast Routing Protocol (DVMRP)
Overview
DVMRP,whichisusedforroutingmulticastswithinasingle,autonomoussystem,isdesignedto
beusedasaninteriorgatewayprotocol(IGP)withinamulticastdomain.Itisadistance‐vector
routingprotocolthatreliesonIGMPfunctionalitytoprovideconnectionlessdatagramdeliveryto
agroupofhostsacrossa
network.
DVMRProutesmulticasttrafficusingatechniqueknownasreversepathforwarding(RPF).When
arouterreceivesIPmulticastpackets,itfirstdoesanRPFchecktodetermineifthepacketsare
receivedonthecorrectinterface. Ifso,therouterforwardsthepacketsouttothefollowing:
•LocalIGMPreceivers
forthatgrouponinterfacesforwhichthetransmittingrouteristhe
designatedforwarder
•Neighborroutersthathaveindicatedtheirdependenceonthetransmittingrouterfor
forwardingmulticastpacketsfromthatsource(thisisdeterminedduringDVMRPRoute
Exchange)andfromwhichthetransmittingrouterhasnotreceivedanyprunemessages.

Ifnot,thepacketsarediscardedbytherouter.Thetransmittingrouterdoesnotforwardthe
packetsbacktothesource.
IfarouterisattachedtoasetofVLANsthatdonotwanttoreceivefromaparticularmulticast
group,theroutercansendaprunemessagebackup
thedistributiontreetostopsubsequent
packetsfromtravelingwheretherearenomembers.DVMRPperiodicallyre‐floodsinorderto
reachanynewhoststhatwanttoreceivefromaparticulargroup.
DVMRProutersdynamicallydiscovertheirneighborsbysendingneighborprobemessages
periodicallytoanIPmulticastgroup
addressthatisreservedforallDVMRProuters.