Specifications

Understanding Multicast
April 16, 2009 Page 11 of 32
PIM,ashared‐treetechnology,designatesarouterastherendezvouspoint(RP),whichistheroot
ofasharedtreeforaparticulargroup.AllsourcessendpacketstothegroupviatheRP(thatis,
trafficflowsfromthesendertotheRP,and fromtheRPtothe
receiver).BymaintainingoneRP‐
rootedtreeinsteadofmultiplesource‐rootedtrees,bandwidthisconserved.
Figure 4illustratesthePIMtrafficflow.
Figure 4 PIM Traffic Flow
1. Thesource’sDRregisters(thatis,encapsulates)andsendsmulticastdatafromthesource
directlytotheRPviaaunicastroutingprotocol(number1infigure).TheRP
de‐encapsulates
eachregistermessageandsendstheresultingmulticastpacketdownthesharedtree.
2. Thelast‐hoprouter(thatis,thereceiver’sDR)sendsamulticastgroup(*,G)joinmessage
upstreamtotheRP,indicatingthatthereceiverwantstoreceivethemulticastdata(number2
infigure).Thisbuilds
theRPtree(RPT)betweenthelast‐hoprouterandtheRP.
3. TheRPsendsanS,Gjoinmessagetothesource(number3infigure).Itmaysendthejoin
messageimmediately,orafterthedatarateexceedsaconfiguredthreshold.Thisallowsthe
administratortocontrolhowPIM‐SM
usesnetworkresources.
4. Thelast‐hoprouterjoinstheshortestpathtree(SPT)andsendsanS,Gjoinmessagetothe
source.(number4infigure).ThisbuildstheSPT.
5. Nativemulticastpackets(thatis,non‐registeredpackets)aresentfromthesource’sDRtothe
receiveronitsSPT(number5
infigure),whileregisteredmulticastpacketscontinuetobesent
fromthesource’sDRtotheRP.
6. Aprunemessageissentfromthelast‐hoproutertotheRP(number6infigure).
3
4
5
6
1
2
Source
Receiver
7
DR RP
Last Hop
Router