Specifications

Understanding Multicast
April 16, 2009 Page 10 of 32
4. Ifthesenderwasadownstreamdependentneighborfromwhichaprunehadpreviouslybeen
received:
• Removestheprunestateforthisneighbor.
•Ifnecessary,updatesanyforwardingcacheentriesbasedonthis(source,group)pairto
includethisdownstreaminterface.
Figure 3showstheDVMR Ppruningandgraftingprocess.
Figure 3 DVMRP Pruning and Grafting
Protocol Independent Multicast (PIM)
Overview
PIMdynamicallybuildsadistributiontreeforforwardingmulticastdataonanetwork.Itis
designedforusewheretheremaybemanydevicescommunicatingatthesametime,andanyone
ofthedevicescouldbethesenderatanyparticulartime.ScenariosforusingPIMmulticasting
includedesktopvideo
conferencingandtelephoneconferencecalls.
PIMreliesonIGMPtechnologytodeterminegroupmembershipsandusesexistingunicastroutes
toperformreversepathforwarding(RPF)checks,whichare,essentially,aroutelookuponthe
source.Itsroutingenginethenreturnsthebestinterface,regardlessofhowtheroutingtableis
constructed.
Inthissense,PIMisindependentofanyroutingprotocol.ItcanperformRPFchecks
usingprotocol‐specificroutes(forexample,OSPFroutes),staticroutes,oracombinationofroute
types.
Multicast
Traffic
Prune*
Prune
Graft
IGMP Join
Source
New Host
DVMRP Multicast
Existing Host
Prune before new
host was added
*