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2. After getting the receiver information, the DR 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 DR to the RP form an RPT branch. Each router on this 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. It also determines whether it still has receivers for that

multicast group. If not, 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 37 Multicast source registration

As shown in Figure 37, the multicast source registers with the RP as follows:

1. The multicast source S sends the first multicast packet to the multicast group G. When receiving the

multicast packet, the source-side DR encapsulates the packet in a PIM register message and

unicasts the message to the RP.

2. After the RP receives the register message, it decapsulates the register message and forwards the

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.

3. The subsequent multicast data from the multicast source are forwarded to the RP along the

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.

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