HP FlexNetwork 7500 Series Configuration Manual page 109

As shown in
When a receiver wants to join the multicast group G, it uses an IGMP message to inform the
1.
receiver-side DR.
After getting the receiver information, the DR sends a join message, which travels hop by hop to
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the RP for the multicast group.
The routers along the path from the DR to the RP form an RPT branch. Each router on this
3.
branch adds to its forwarding table a (*, G) entry, where the asterisk (*) represents any multicast
source. The RP is the root of the RPT, and the DR is a leaf of the RPT.
When the multicast data addressed to the multicast group G reaches the RP, the RP forwards the
data to the DR along the established RPT, and finally to the receiver.
When a receiver is no longer interested in the multicast data addressed to the multicast group G, the
receiver-side DR sends a prune message. The prune message goes hop by hop along the RPT to
the RP. After receiving the prune message, the upstream node deletes the interface that connects to
this downstream node from the outgoing interface list. At the same time, the upstream router checks
for 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.
Multicast source registration
The multicast source uses the registration process to inform an RP of its presence.
Figure 39 Multicast source registration
As shown in
The multicast source S sends the first multicast packet to the multicast group G. When
1.
receiving the multicast packet, the source-side DR encapsulates the packet into a PIM register
message and unicasts the message to the RP.
After the RP receives the register message, it decapsulates the register message and forwards
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the register message down to the RPT. Meanwhile, it sends an (S, G) source-specific join
message toward the multicast source. The routers along the path from the RP to the multicast
source constitute an SPT branch. Each router on this branch creates an (S, G) entry in its
forwarding table.
The subsequent multicast data from the multicast source are forwarded to the RP along the
3.
established SPT. When the multicast data reaches the RP along the SPT, the RP forwards the
data to the receivers along the RPT. Meanwhile, it unicasts a register-stop message to the
source-side DR to prevent the DR from unnecessarily encapsulating the data.
Figure 38, the process of building an RPT is as follows:
Figure 39, the multicast source registers with the RP as follows:
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