5-2MSDP achieves this objective. By establishing MSDP peer relationships among RPs of differentPIM-SM domains, source active (SA) messages can be forwarded among domains and the multicastsource information can be shared.z MSDP is applicable only if the intra-domain multicast protocol is PIM-SM.z MSDP is meaningful only for the any-source multicast (ASM) model.How MSDP WorksMSDP peersWith one or more pairs of MSDP peers configured in the network, an MSDP interconnection map isformed, where the RPs of different PIM-SM domains are interconnected in series. Relayed by theseMSDP peers, an SA message sent by an RP can be delivered to all other RPs.Figure 5-1 Where MSDP peers are in the networkAs shown in Figure 5-1 , an MSDP peer can be created on any PIM-SM router. MSDP peers created onPIM-SM routers that assume different roles function differently.1) MSDP peers on RPsz Source-side MSDP peer: the MSDP peer nearest to the multicast source (Source), typically thesource-side RP, like RP 1. The source-side RP creates SA messages and sends the messages toits remote MSDP peer to notify the MSDP peer of the locally registered multicast sourceinformation. A source-side MSDP must be created on the source-side RP; otherwise it will not beable to advertise the multicast source information out of the PIM-SM domain.z Receiver-side MSDP peer: the MSDP peer nearest to the receivers, typically the source-side RP,like RP 3. Upon receiving an SA message, the receiver-side MSDP peer resolves the multicastsource information carried in the message and joins the SPT rooted at the source across thePIM-SM domain. When multicast data from the multicast source arrives, the receiver-side MSDPpeer forwards the data to the receivers along the RPT.
