3Com 7757 Configuration Manual page 483

Implementing inter-domain multicast delivery by leveraging MSDP peers
As shown in Figure 121, 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 wish to know the specific
location of Source so that receiver hosts can receive multicast traffic originated
from it, MSDP peering relationships should be established between RP 1 and RP 3
and between RP 3 and RP 2 respectively.
Figure 121 MSDP peering relationships
MSDP peers
Multicast packets
SA message
Join message
Register message
DR 1
Source
RP 1
PIM-SM 1
The process if 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.
4 SA messages are forwarded from one MSDP peer to another, and finally the
information of the multicast source traverses all PIM-SM domains with MSDP
peers (PIM-SM 2 and PIM-SM 3 in this example).
MSDP Overview
Receiver
RP 2
PIM-SM 4
PIM-SM 3
483
DR 2
PIM-SM 2
RP 3