MSDP

Overview

MSDP is a protocol that enables multiple PIM-SM domains to communicate with each other using their own RPs. MSDP also enables multiple RPs in a single PIM-SM domain to establish MSDP mesh-groups and to synchronize information between anycast RPs about the active sources being served by each anycast RP peer. The 7950 XRS, 7750 SR, and the 7450 ESS in mixed mode support MSDP.

Each PIM-SM domain has its own RPs and MSDP enables these RPs to inform each other about active sources. When an active source is detected, the RPs send PIM-SM explicit join messages to the active source. When RPs in remote domains know about active sources, they can pass on this information to their local receivers and multicast data can be forwarded between the domains.

The RP learns about a new multicast source within its domain through the PIM register mechanism and encapsulates the first data packet in a source active message. After an RPF check, the MSDP source active message is flooded by each peer to its MSDP peers until it reaches every MSDP router in the interconnected networks. If the receiving MSDP peer is an RP, and the RP has a (*,G) entry for the group in the source active message, the message is accepted. The RP creates an (S,G) state for the source and join to the shortest path tree for the state of the source.

The encapsulated data is forwarded down the RP shared tree. When the packet is received by a receiver’s last hop router, the last-hop may also join the shortest path tree to the source.

The following figure shows a sample of how data flows from a source in Domain A to a receiver in Domain E in an MSDP implementation for PIM-SM.

Figure 28-5: Sample implementation of MSDP for PIM-SM