3Com Switch 8800 Advanced Software V5 Configuration Guide

570 CHAPTER 42: PIM CONFIGURATION
As shown in Figure 178, host B and host C are receivers of multicast data. 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.
Figure 179 Multicast registration
As shown in Figure 179, the multicast source registers with the RP as follows:
1 When the multicast source S sends the first multicast packet to a multicast
group G, the DR directly connected with the multicast source, upon
Source
Server
Host A
Host B
Host C
Receiver
Receiver
Multicast packets
SPT
Join message
Register message
RPDR