HP MSR2003 Configuration Manual page 150

Figure 49 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. This register message is forwarded to
RP 1, and RP 1 obtains the multicast source information from the message.
2. As the source-side RP, RP 1 creates SA messages and periodically sends them to its MSDP peer.
An SA message contains the addresses of the multicast source (S), the multicast group (G), and the
RP that has created this SA message (RP 1, in this example).
3. On MSDP peers, each SA message undergoes an RPF check and multicast policy-based filtering.
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 to avoid SA message flooding between MSDP peers.
NOTE:
An MSDP mesh group refers to a group of MSDP peers that establish MSDP peering relationships with
each other and share the same group name.
4. SA messages are forwarded from one MSDP peer to another. Finally, information about the
multicast source traverses all PIM-SM domains with MSDP peers (PIM-SM 2 and PIM-SM 3, in this
example).
5. After receiving the SA message that RP 1 created, RP 2 in PIM-SM 2 examines whether any
receivers for the multicast group exist in the domain.
If a receiver exists in the domain, the RPT for the multicast group G is maintained between RP 2
and the receivers. RP 2 creates an (S, G) entry and sends an (S, G) join message. The join
message travels hop by hop toward the multicast source, and the SPT is established across the
PIM-SM domains.
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