MSDP Peers

The first task for implementing inter-domain multicast is to establish MSDP peer relationships.

RPs in PIM-SM domains set up MSDP peer relationships. MSDP peers exchange SA messages that carry (S, G) information generated when source DRs register to the RPs. Message exchange among MSDP peers ensures that SA messages sent by any RP can be received by all the other RPs.

As shown in Figure 6-1, MSDP can be deployed on other PIM routers apart from the RPs. MSDP peers established on different PIM routers have different functions.

Figure 6-1 Locations of MSDP peers

• Establish MSDP peers on RPs.

  Classification	Location	Function
  MSDP peer on the source end	Closest to the multicast source (generally the source RP, such as RP1)	The source RP creates SA messages, sends SA messages to remote MSDP peers, and advertises multicast source information registered to the local RP. The MSDP peer on the source end must be configured on an RP. Otherwise, the MSDP peer on the source end cannot advertise multicast source information.
  MSDP peer on the receiver end	Closest to the receiver (such as RP3)	After receiving SA messages, the MSDP peer on the receiver end joins an SPT with the multicast source being the root according to source information contained in SA messages. After receiving multicast data from this source, the peer forwards multicast data along the rendezvous point tree (RPT) to local receivers. The MSDP peer on the receiver end must be configured on an RP. Otherwise, the MSDP peer on the receiver end cannot receive multicast source information from other domains.
  Intermediate MSDP peer	Having multiple remote MSDP peers (such as RP2)	The intermediate MSDP peer forwards SA messages received from a remote MSDP peer to other remote MSDP peers.

• Establish MSDP peers on common PIM routers but not the RPs.

  These MSDP peers (such as RouterA and RouterB) only forward SA messages they receive.

To ensure that all RPs on a network share source information and the number of devices configured with MSDP is minimized, it is recommended that you configure MSDP only on the RPs on the network.