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RPT building
Figure 44 RPT building in a PIM-SM domain
As shown in
, the process of building an RPT is as follows:
1.
When a receiver joins multicast group G, it uses an IGMP message to inform the directly
connected DR.
2.
After getting the receiver information, the DR sends a join message, which is forwarded, hop by
hop, to the RP that corresponds to the multicast group.
3.
The routers along the path from the DR to the RP form an RPT branch. Each router on this
branch generates a (*, G) entry in its forwarding table. The asterisk (*) means any multicast
source. The RP is the root of the RPT, and the DRs are the leaves of the RPT.
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. 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
In multicast source registration, the multicast source informs the RP of its existence.
Source
Server
Host A
Host B
Host C
Receiver
Receiver
Multicast packets
RPT
Join message
RP
DR
DR
