3Com S7906E Configuration Manual page 1086

2) MSDP peers created on common PIM-SM routers (other than RPs)
Router A and Router B are MSDP peers on common multicast routers. Such MSDP peers just forward
received SA messages.
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 relationships should be built among all C-RPs so that the winner
C-RP is always on the "MSDP interconnection map", while loser C-RPs 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 1-2, 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 1-2 MSDP peering relationships
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.
Receiver
RP 2
PIM-SM 4
PIM-SM 3
1-3
DR 2
PIM-SM 2
RP 3