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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 DR 1 in PIM-SM 1 receives the first multicast packet from the multicast source to
multicast group G, DR 1 encapsulates the multicast data within a register message. It sends the
register message to RP 1, and RP 1 obtains 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 (that is, RP 1).
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 so as to avoid flooding of SA messages
between MSDP peers.
4.
SA messages are forwarded from one MSDP peer to another. The information about the
multicast source travels across 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 for the multicast group 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 hop by hop toward DR 1 at the multicast source side, so that it can directly
join the SPT rooted at the source over other PIM-SM domains. Then, the multicast data can
flow along the SPT to RP 2 and RP 2 can forward the data to the receivers along the RPT.
After receiving the multicast traffic, the DR at the receiver side (DR 2) determines whether to
initiate an RPT-to-SPT switchover process.
{
If receivers for the group do not exist in the domain, RP 2 neither creates an (S, G) entry nor
joins the SPT rooted at the source.
When using MSDP for inter-domain multicasting, once an RP receives information form a multicast
source, it no longer relies on RPs in other PIM-SM domains. The receivers can override the RPs in
other domains and directly join the multicast source-based SPT.
