3Com MSR 50 Series Configuration Manual page 1133

will assume the role of common PIM-SM routers on the "MSDP interconnection
map".
Implementing inter-domain multicast delivery by leveraging MSDP peers
As shown in Figure 334, an active source exists in the domain PIM-SM 1, and RP 1
learns the existence of this number source through multicast source registration. If
PIM-SM 2 and PIM-SM 3 also wish to know the specific location of the multicast
source so as to receive multicast traffic originated from it, MSDP peering
relationships should be established between RP1 and RP3 and between RP 3 and
RP 2 respectively.
Figure 334 MSDP peers
MSDP peers
Multicast packets
SA message
Join message
Register message
DR 1
Source
RP 1
PIM-SM 1
The process of implementing 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 gets aware of 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 which has created this SA
message (namely RP 1).
3 On MSDP peers, each SA message is subject to 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. In addition, you can configure MSDP
peers into an MSDP mesh group so as to avoid flooding of SA messages between
MSDP peers.
MSDP Overview
Receiver
RP 2
PIM-SM 4
PIM-SM 3
1133
DR 2
PIM-SM 2
RP 3