H3C Technologies H3C S3600 Series Switches - Manual (Page 435)

5-4
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).
5) Upon receiving the SA message create by RP 1, RP 2 in PIM-SM 2 checks whether there are any
receivers for the multicast group in the domain.
z If so, 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 towards 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 is forwarded by RP 2 to the
receivers along the RPT. Upon receiving the multicast traffic, the DR at the receiver side (DR 2)
decides whether to initiate an RPT-to-SPT switchover process.
z If no receivers for the group exist in the domain, RP 2 does dot create an (S, G) entry and does join
the SPT rooted at the source.
z An MSDP mesh group refers to a group of MSDP peers that have MSDP peering relationships
among one another and share the same group name.
z 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.
RPF check rules for SA messages
As shown in Figure 5-3 , there are five autonomous systems in the network, AS 1 through AS 5, with IGP
enabled on routers within each AS and EBGP as the interoperation protocol among different ASs. Each
AS contains at least one PIM-SM domain and each PIM-SM domain contains one ore more RPs. MSDP
peering relationships have been established among different RPs. RP 3, RP 4 and RP 5 are in an
MSDP mesh group. On RP 7, RP 6 is configured as its static RPF peer.