PIM Overview 635
domain, and the position of the RP corresponding to each multicast group is
calculated through the BSR mechanism.
Figure 190 shows the positions of C-RPs and the BSR in the network.
Figure 190 BSR and C-RPs
RPT establishment
Figure 191 RPT establishment in a PIM-SM domain
As shown in Figure 191, the process of building an RPT is as follows:
1 When a receiver joins a multicast group G, it uses an IGMP message to inform the
directly connected DR.
2 Upon getting the receiver information, the DR sends a join message, which is hop
by hop forwarded to the RP corresponding 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 * means any
multicast source. The RP is the root, while the DRs are the leaves, of the RPT.
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BSR message
PIM-SM
BSR
C-RP
C-RP
C-RP
C-BSR
Source
Server
Host A
Host B
Host C
Receiver
Receiver
Multicast packets
RPT
Join message
RP DR
DR
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