3Com 7757 Configuration Manual page 468

468 C 47: PIM C
HAPTER ONFIGURATION
Figure 116 Diagram for RPT building in PIM-SM
Source
Server
RPT
Join message
Multicast packets
Each router on the path from the leaf router to RP will generate (*, G) entries in
the forwarding table. The routers on the path forms a branch of RPT. A (*, G) entry
represents the information from any source to the multicast group G. RP is the
root of RPT and the receivers are leaves of RPT.
When the packet from the multicast source S to the multicast group G passes by
RP, the packet will reach the leaf router and receiver host along the established
path in RPT.
When the receiver is not interested in the multicast information any more, the
multicast router nearest to the receiver will send Prune messages to RP hop by hop
in the direction reverse to RPT. When the first upstream router receives the Prune
message, it will delete the links with the downstream routers from the interface
list and check whether it has the receivers interested in the multicast information.
If not, the upstream router will continue to forward the Prune message to
upstream routers.
Multicast source registration
In order to inform RP about the existence of multicast source S, when multicast
source S sends a multicast packet to the multicast group G, the router directly
connected to S will encapsulate the received packet into a registration packet and
send it to the corresponding RP in unicast form, as shown in Figure 117:
RP
Host A
Receiver
DR
Host B
DR
Receiver
Host C