5-3An RP is dynamically elected from C-RPs. To enhance network robustness, a PIM-SM network typicallyhas more than one C-RP. As the RP election result is unpredictable, MSDP peering relationshipsshould 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 interconnectionmap”.Implementing inter-domain multicast delivery by leveraging MSDP peersAs shown in Figure 5-2, an active source (Source) exists in the domain PIM-SM 1, and RP 1 haslearned the existence of Source through multicast source registration. If RPs in PIM-SM 2 and PIM-SM3 also wish to know the specific location of Source so that receiver hosts can receive multicast trafficoriginated from it, MSDP peering relationships should be established between RP 1 and RP 3 andbetween RP 3 and RP 2 respectively.Figure 5-2 MSDP peering relationshipsRP 1DR 1SourcePIM-SM 1 PIM-SM 3PIM-SM 2PIM-SM 4RP 3RP 2DR 2MSDP peersSA messageJoin messageMulticast packetsRegister messageReceiverThe 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 1encapsulates 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 itsMSDP 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 andmulticast policy–based filtering, so that only SA messages that have arrived along the correct pathand passed the filtering are received and forwarded. This avoids delivery loops of SA messages. In
