3Com S7902E, Configuration Manual

Page: 1031 / 2621

1-9
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 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 the interface connected 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 1-6 Multicast source registration
Source
Server
Host A
Host B
Host C
Receiver
Receiver
Multicast packets
SPT
Join message
Register message
RP DR
As shown in Figure 1-6 , the multicast source registers with the RP as follows:
1) When the multicast source S sends the first multicast packet to multicast group G, the DR directly
connected with the multicast source, upon receiving the multicast packet, encapsulates the packet
in a PIM register message, and sends the message to the corresponding RP by unicast.
2) When the RP receives the register message, it extracts the multicast packet from the register
message and forwards the multicast packet down the RPT, and sends an (S, G) join message hop
by hop toward the multicast source. Thus, the routers along the path from the RP to the multicast
source constitute an SPT branch. Each router on this branch generates an (S, G) entry in its
forwarding table. The DR at the multicast source side is the root, while the RP is the leaf, of the
SPT.
3) The subsequent multicast data from the multicast source travels along the established SPT to the
RP, and then the RP forwards the data along the RPT to the receivers. When the multicast traffic
arrives at the RP along the SPT, the RP sends a register-stop message to the source-side DR by
unicast to stop the source registration process.