97
Figure 38 RPT building at the receiver side
As shown in Figure 38, the process for building a receiver-side RPT is the same as the process for
building an RPT in PIM-SM:
1. When a receiver wants to join the multicast group G, it uses an IGMP message to inform the
directly connected router.
2. After receiving the message, the router sends a join message, which is forwarded hop by hop to
the RP for the multicast group.
3. The routers along the path from the receiver's directly connected router to the RP form an RPT
branch. Each router on this branch adds a (*, G) entry to its forwarding table.
After a receiver host leaves the multicast group G, the directly connected router multicasts a prune
message to all PIM routers on the subnet. The prune message goes hop by hop along the reverse
direction of the RPT to the RP. After receiving the prune message, an upstream node removes the
interface that connects to the downstream node from the outgoing interface list. At the same time,
the upstream router checks the existence of receivers for that multicast group. If no receivers for the
multicast group exist, the router continues to forward the prune message to its upstream router.
Source
Server A
Server B
Host B
Host C
Receiver
Receiver
Multicast packets
Receiver-side RPT
Join message
RP
Source
Host A
Receiver
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