636 CHAPTER 47: PIM CONFIGURATION
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 192 Multicast registration
As shown in Figure 192, 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 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
multicast source 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.
Source
Server
Host A
Host B
Host C
Receiver
Receiver
Multicast packets
SPT
Join message
Register message
RPDR
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