How MSDP works

	NOTE: For more information about DRs, BSRs, C-BSRs, RPs, C-RPs, SPTs, and RPTs, see "Configuring PIM."

MSDP peer relationships are established between appropriate devices on the network, so that the RPs of different PIM-SM domains are interconnected with one another. These MSDP peers exchange source active (SA) messages, so that the multicast source information is shared among these domains.

One or more pairs of MSDP peers on the network form an MSDP interconnection map.

As shown in Figure 60, an MSDP peer can be created on any PIM-SM device. MSDP peers created on PIM-SM devices that assume different roles function differently.

• MSDP peers created on RPs:

  • Source-side MSDP peer—MSDP peer closest to the multicast source, such as RP 1. The source-side RP creates and sends SA messages to its remote MSDP peer to notify the MSDP peer of the locally registered multicast source information.

    A source-side MSDP peer must be created on the source-side RP. Otherwise, it cannot advertise the multicast source information out of the PIM-SM domain.

  • Receiver-side MSDP peer—MSDP peer closest to the receivers, typically the receiver-side RP, such as RP 3. After receiving an SA message, the receiver-side MSDP peer resolves the multicast source information carried in the message. Then, it joins the SPT rooted at the multicast source across the PIM-SM domains. When multicast data from the multicast source arrives, the receiver-side MSDP peer forwards the data to the receivers along the RPT.

  • Intermediate MSDP peer—MSDP peer with multiple remote MSDP peers, such as RP 2. An intermediate MSDP peer forwards SA messages received from one remote MSDP peer to other remote MSDP peers. It acts as a relay for forwarding multicast source information.

• MSDP peers created on PIM-SM devices that are not RPs:

  Device A and Device B are MSDP peers on multicast devices that are not RPs. Such MSDP peers only forward SA messages.

In a PIM-SM network using the BSR mechanism, the RP is dynamically elected from C-RPs. A PIM-SM network typically has multiple C-RPs to ensure network robustness. Because the RP election result is unpredictable, MSDP peering relationships must be built between all C-RPs to always keep the winning C-RP on the MSDP interconnection map. Losing C-RPs assume the role of common PIM-SM devices on this map.

As shown in Figure 61, an active source (Source) exists in the domain PIM-SM 1, and RP 1 has learned the existence of Source through multicast source registration. RPs in PIM-SM 2 and PIM-SM 3 also seek the location of Source so that multicast traffic from Source can be sent to their receivers. MSDP peering relationships must be established between RP 1 and RP 3 and between RP 3 and RP 2.

The process of implementing PIM-SM 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 1 encapsulates the data within a register message. It sends the register message to RP 1, and RP 1 obtains information about the multicast source.

2. As the source-side RP, RP 1 creates SA messages and periodically sends them to its MSDP peer.

   An SA message contains the address of the multicast source (S), the multicast group address (G), and the address of the RP that has created this SA message (RP 1, in this example).

3. On MSDP peers, each SA message undergoes an RPF check and multicast policy-based filtering. 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 to avoid SA message flooding between MSDP peers. An MSDP mesh group refers to a group of MSDP peers that establish MSDP peering relationships with each other and share the same group name.

4. SA messages are forwarded from one MSDP peer to another. Finally, information about the multicast source traverses all PIM-SM domains with MSDP peers (PIM-SM 2 and PIM-SM 3, in this example).

5. After receiving the SA message that RP 1 created, RP 2 in PIM-SM 2 examines whether any receivers for the multicast group exist in the domain.

   • If a receiver exists in the domain, 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. The join message travels hop by hop toward the multicast source, and the SPT is established across the PIM-SM domains.

     The subsequent multicast data flows to RP 2 along the SPT, and from RP 2 to the receiver-side DR along the RPT. After receiving the multicast data, the receiver-side DR determines whether to initiate an RPT-to-SPT switchover process based on its configuration.

   • If no receivers exist in the domain, RP 2 neither creates an (S, G) entry nor sends a join message toward the multicast source.

In inter-domain multicasting using MSDP, once an RP gets information about a multicast source in another PIM-SM domain, 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 SPT rooted at the source.

The MSDP peer-RPF check is used for forwarding SA messages on a network that runs MSDP. If the peer-RPF check succeeds, the SA message is accepted and forwarded. Otherwise, the SA message is discarded.

As shown in Figure 62:

• There are five ASs on the network. IGP runs within each AS, and BGP or MBGP runs between these ASs.

• Each AS contains a minimum of one PIM-SM domain, and each PIM-SM domain contains a minimum of one RP.

• MSDP peering relationship has been established among these RPs.

  RP 3, RP 4, and RP 5 are in the same MSDP mesh group.

  RP 6 is configured as the static RPF peer of RP 7.

  Figure 62: MSDP peer-RPF forwarding

  

The process of peer-RPF forwarding is as follows:

1. RP 1 creates an SA message and forwards it to its peer RP 2.

2. RP 2 determines that RP 1 is the RP that creates the SA message because the RP address in the SA message is the same as that of RP 1. Then, RP 2 accepts and forwards the SA message.

3. RP 3 accepts and forwards the SA message, because RP 2 and RP 3 reside in the same AS and RP 2 is the next hop of RP 3 to RP 1.

4. RP 4 and RP 5 accept the SA message, because RP 3 is in the same mesh group with them. Then, RP 4 and RP 5 forward the SA message to their peer RP 6 rather than other members of the mesh group.

5. RP 4 and RP 5 reside in the closest AS in the route to RP 1. However, RP 6 accepts and forwards only the SA message from RP 5, because the IP address of RP 5 is higher than that of RP 4.

6. RP 7 accepts and forwards the SA message, because RP 6 is its static RPF peer.

7. RP 8 accepts and forwards the SA message, because RP 7 is the EBGP or MBGP next hop of the peer-RPF route to RP 1.

8. RP 9 accepts the SA message, because RP 8 is the only RP of RP 9.