Specifications

Understanding Multicast
April 16, 2009 Page 13 of 32
•Register‐Stop—ThesemessagesareusedbytheRPtotellthesource’sDRtostopregistering
trafficforaparticularsource.
• Join/Prune(J/P)—Thesemessagescontaininformationongroupmembershipreceivedfrom
downstreamrouters.
PIM‐SMadoptsRPFtechnologyinthejoin/pruneprocess.Whenamulticastpacketarrives,
therouterfirstjudgesthecorrectnessofthearrivinginterfaces:
–Ifthepacketisasourceaddress/multicastgroup(S,G)entry(ontheshortestpath tree
(SPT)),thenthecorrectinterfaceisthereversepathforwarding(RPF)interfacetowards
thesource.
–IfthepacketisnotanS,Gentry(ontheRP
tree(RPT)),thenthecorrectinterfaceisthe
RPFinterfacetowardstheRP.
ArouterdirectlyconnectedtothehostsisoftenreferredtoasaleafrouterorDR.Theleaf
routerisresponsibleforsendingtheprunemessagestothe RP,inf ormingittostopsending
multicastpacketsassociated
withaspecificmulticastgroup.WhentheRPreceivestheprune
message,itwillnolongerforwardthemulticasttrafficouttheinterfaceonwhichitreceived
theprunemessage.
• Assert—Thesemessagesindicatethatthedevicereceivedadatapacketonitsoutbound
(receiving)interfaceforthegroup.They
reportthemetricordistancetothesourceorRPto
helpthedeviceidentifythemostdirectpathtotherootofthetree.Ifmultipleroutersclaimto
havethemostdirectpathtothesourceorRP,eachdevicesendsitsownassertmessageand
therouter
withthebestmetricwins.Theotherdevicewillthenremovethatlinkfromits
outboundinterfacelistforthegroup.
• Bootstrap—ThesemessagesaresentbythePIM‐SMrouterthathasbeenelectedasthe
bootstraprouter(BSR)toinformallPIM‐SMroutesoftheRP/groupmappings.
• Candidate
RPmessage—ThesemessagesaresentbytheconfiguredcandidateRProutersto
theBSRtoinformtheBSRofitsRP/groupcandidacy.
PIM Terms and Definitions
Table 1liststermsanddefinitionsusedinPIMconfiguration.
Table 1 PIM Terms and Definitions
Term Definition
Bootstrap Router (BSR) A PIM router responsible for collecting, within a PIM domain, the set of potential
rendezvous points (RPs) and distributing the RP set information to all PIM
routers within the domain. The BSR is dynamically elected from the set of
candidate BSRs.
RP set information includes group-to-RP mappings.
Candidate Bootstrap
Router (Candidate-BSR)
A small number of routers within a PIM domain are configured as candidate
BSRs, and each C-BSR is given a BSR priority. All C-BSRs multicast bootstrap
messages (BSMs) containing their priority to the ALL-PIM-ROUTERS group.
When a C-BSR receives a bootstrap message from a C-BSR with a higher
priority, it stops sending. This continues until only one C-BSR remains sending
bootstrap messages, and it becomes the elected BSR for the domain.