When multicast data for multicast group G is sent to RP, it will travels along the constructed
RPT to DR and finally arrives at the receivers.
When a receiver is no longer interested in the multicast group data, its directly connected DR
will send prune message up the RPT toward the group’s corresponding RP; after the upstream
node receives this prune message, it will delete the link to the downstream node in its interface
list and check if there are other receivers of this group. If there are no more receivers, the
prune message will be sent upstream.
Multicast Source Registering
The multicast source register is to inform its presence to the RP.
As shown in Figure 11-18, the process of the multicast source registering to RP is illustrated
below:
Figure 11-16 Multicast Source Register Topology in PIM SM
(1)
When the multicast source S’s directly connected DR receives a multicast packet sent
from the multicast source to the multicast group G, the DR will encapsulate this packet into
a register packet and send it to the corresponding RP in unicast way;
(2)
After the RP receives the register packet, it will de-capsulate this packet and send the
packaged multicast data to the receivers along the RPT, and meanwhile it will send join
message to the multicast source hop-by-hop. The join message travels router-by-router
toward the source from the RP, constructing a branch of the SPT as it goes. These routers
generate (S, G) entries in their forwarding tables. The SPT works with multicast source as
the root, and RP as the branch.
(3)
The multicast data sent from the multicast source travels along the constructed SPT to RP,
and is forwarded by the RP to the receivers along the RPT. When RP receives the multicast
data from the RPT, it will send Register-Stop Message to the DR directly connected to the
multicast source to finish the multicast source register process.
279
