3com S7906E Instruccion De Instalación

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. 

The purpose of multicast source registration is to inform the RP about the existence of the multicast 

source. 

Figure 1-6 Multicast source registration 

Server

Host A

Host B

Host C

Multicast packets

SPT

Join message

Register message

 

 

As shown in 

, 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.