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In a PIM-SM network running the BSR mechanism, the RP is dynamically elected from C-RPs. To enhance
network robustness, a PIM-SM network typically has more than one C-RP. As the RP election result is
unpredictable, MSDP peering relationship must be built among all C-RPs so that the winner C-RP is
always on the "MSDP interconnection map," and loser C-RPs will assume the role of common PIM-SM
routers on the "MSDP interconnection map."
Inter-domain multicast delivery through MSDP
As shown in Figure 59, an active source (Source) exists in the domain PIM-SM 1, and RP 1 has learned
the existence of Source through multicast source registration. If RPs in PIM-SM 2 and PIM-SM 3 also seek
the specific location of Source so that receiver hosts can receive multicast traffic that the source sends,
H3C recommends you to establish MSDP peering relationship between RP 1 and RP 3 and between RP
3 and RP 2, respectively.
Figure 59 Inter-domain multicast delivery through MSDP
The process of implementing PIM-SM inter-domain multicast delivery by leveraging MSDP peers is as
follows:
1. When the multicast source in PIM-SM 1 sends the first multicast packet to multicast group G, DR 1
encapsulates the multicast data within a register message and sends the register message to RP 1.
Then, RP 1 identifies the information related to the multicast source.
2. As the source-side RP, RP 1 creates SA messages and periodically sends the SA messages to its
MSDP peer. An SA message contains the source address (S), the multicast group address (G), and
the address of the RP that has created this SA message (namely, RP 1).
3. On MSDP peers, each SA message undergoes a reverse path forwarding (RPF) check and
multicast policy–based filtering, so that only SA messages that have arrived along the correct path
and passed the filtering are received and forwarded. This avoids delivery loops of SA messages.
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