3Com 5500-EI PWR Install Manual page 375

Figure 4-5 Building an RPT in PIM-SM RPT building
As shown in Figure 4-5, the process of building an RPT is as follows:
1) When a receiver joins a multicast group G, it uses an IGMP message to inform the directly
connected DR.
2) Upon getting the receiver information, the DR sends a join message, which is hop by hop
forwarded to the RP corresponding to the multicast group.
3) The routers along the path from the DR to the RP form an RPT branch. Each router on this branch
generates a (*, G) entry in its forwarding table. The * means any multicast source. The RP is the
root, while the DRs are the leaves, of the RPT.
The multicast data addressed to the multicast group G flows through the RP, reaches the corresponding
DR along the established RPT, and finally is delivered to the receiver.
When a receiver is no longer interested in the multicast data addressed to a multicast group G, the
directly connected DR sends a prune message, which goes hop by hop along the RPT to the RP. Upon
receiving the prune message, the upstream node deletes its link with this downstream node from the
outgoing interface list and checks whether it itself has receivers for that multicast group. If not, the router
continues to forward the prune message to its upstream router.
Multicast source registration
The purpose of multicast source registration is to inform the RP about the existence of the multicast
source.
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