Multicast Source Discovery Protocol (MSDP)

In the PIM sparse mode model, the router that’s closest to the sources or receivers registers with the RP. The RP knows about all the sources and receivers for any particular group. Network administrators may want to configure several RPs and create several PIM-SM domains. In each domain, RPs have no way of knowing about sources located in other domains. MSDP is an elegant way to solve this problem.

MSDP was developed for peering between Internet service providers (ISPs). ISPs did not want to rely on an RP maintained by a competing ISP to provide service to their customers. MSDP allows each ISP to have its own local RP and still forward and receive multicast traffic to the Internet.

MSDP enables RPs to share information about active sources. RPs know about the receivers in their local domain. When RPs in remote domains hear about the active sources, they can pass on that information to their local receivers and multicast data can then be forwarded between the domains. A useful feature of MSDP is that it allows each domain to maintain an independent RP that does not rely on other domains. MSDP gives the network administrators the option of selectively forwarding multicast traffic between domains or blocking particular groups or sources. PIM-SM is used to forward the traffic between the multicast domains.

The RP in each domain establishes an MSDP peering session using a TCP connection with the RPs in other domains or with border routers leading to the other domains. When the RP learns about a new multicast source within its own domain (through the normal PIM register mechanism), the RP encapsulates the first data packet in a Source-Active (SA) message and sends the SA to all MSDP peers. MSDP uses a modified RPF check in determining which peers should be forwarded the SA messages. This modified RPF check is done at an AS level instead of a hop-by-hop metric. The SA is forwarded by each receiving peer, also using the same modified RPF check, until the SA reaches every MSDP router in the internetwork—theoretically, the entire multicast Internet. If the receiving MSDP peer is an RP, and the RP has an (*, G) entry for the group in the SA (that is, there is an interested receiver), the RP creates the (S, G) state for the source and joins the shortest path tree for the source. The encapsulated data is decapsulated and forwarded down the shared tree of that RP. When the packet is received by the last-hop router of the receiver, the last-hop router also may join the shortest path tree to the source. The MSDP speaker periodically sends SAs that include all sources within the domain of the RP.

When a receiver joins a group that is transmitted by a source in another domain, the RP sends PIM join messages in the direction of the source to build a shortest path tree. The DR sends packets on the source tree within the source domain, which can travel through the RP in the source domain and along the branches of the source tree to other domains. In domains where there are receivers, RPs in those domains can be on the source tree. The peering relationship is conducted over a TCP connection.

Figure 6-15 shows four PIM domains. The connected RPs (routers) are called MSDP peers because they are exchanging active source information with each other. Each MSDP peer advertises its own set of multicast source information to the other peers. Source host 2 sends the multicast data to group 224.1.1.1. On RP 6, the MSDP process learns about the source through PIM register messages and generates Source-Active (SA) messages to its MSDP peers that contain information about the sources in its domain. When RP 3 and RP 5 receive the SA messages, they forward these messages to their MSDP peers. When RP 5 receives the request from host 1 for the multicast data on group 224.1.1.1, it builds a shortest path tree to the source by sending a PIM join message in the direction of host 2 at 192.1.1.1.

  

Figure 6-15 MSDP Peering Between RPs in Different PIM Domains