Configuring Multicast Source Discovery Protocol

The Multicast Source Discovery Protocol (MSDP) is used by Protocol Independent Multicast (PIM) Sparse devices to exchange source information across PIM Sparse domains. Devices running MSDP can discover PIM Sparse sources in other PIM Sparse domains.

The following figure shows an example of some PIM Sparse domains. For simplicity, this example shows one Designated Router (DR), one group source, and one receiver for the group. Only one PIM Sparse device within each domain needs to run MSDP.

Figure 1. PIM Sparse domains joined by MSDP devices

In this example, the source for PIM Sparse multicast group 232.0.1.95 is in PIM Sparse domain 1. The source sends a packet for the group to its directly attached DR. The DR sends a PIM register message for this flow to the RPDR. The RP is configured for MSDP, which enables the RP to exchange source information with other PIM Sparse domains by communicating with RPs in other domains that are running MSDP.

The RP sends the source information to each peer through a Source Active message. The message contains the IP address of the source, the group address to which the source is sending, and the IP address of the RP.

In this example, the Source Active message contains the following information:

  • Source address: 206.251.14.22
  • Group address: 232.1.0.95
  • RP address: 206.251.17.41

Figure 1 shows only one peer for the MSDP device (which is also the RP here) in domain 1, so the Source Active message goes to only that peer. When an MSDP device has multiple peers, it sends a Source Active message to each of those peers. Each peer sends the Source Advertisement to other MSDP peers. The RP that receives the Source Active message also sends a Join message to the source if the RP that received the message has receivers for the group and source.