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Understanding MSDP

The Multicast Source Discovery Protocol (MSDP) is used to connect multicast routing domains. It typically runs on the same router as the Protocol Independent Multicast (PIM) sparse-mode rendezvous point (RP). Each MSDP router establishes adjacencies with internal and external MSDP peers similar to the way BGP establishes peers. These peer routers inform each other about active sources within the domain. When they detect active sources, the routers can send PIM sparse-mode explicit join messages to the active source.

The peer with the higher IP address passively listens to a well-known port number and waits for the side with the lower IP address to establish a Transmission Control Protocol (TCP) connection. When a PIM sparse-mode RP that is running MSDP becomes aware of a new local source, it sends source-active type, length, and values (TLVs) to its MSDP peers. When a source-active TLV is received, a peer-reverse-path-forwarding (peer-RPF) check (not the same as a multicast RPF check) is done to make sure that this peer is in the path that leads back to the originating RP. If not, the source-active TLV is dropped. This TLV is counted as a “rejected” source-active message.

The MSDP peer-RPF check is different from the normal RPF checks done by non-MSDP multicast routers. The goal of the peer-RPF check is to stop source-active messages from looping. Router R accepts source-active messages originated by Router S only from neighbor Router N or an MSDP mesh group member. For more information about configuring MSDP mesh groups, see Example: Configuring MSDP with Active Source Limits and Mesh Groups.

Router R locates its MSDP peer-RPF neighbor (Router N) deterministically. A series of rules is applied in a particular order to received source-active messages, and the first rule that applies determines the peer-RPF neighbor. All source-active messages from other routers are rejected.

1. If Router X originated the source-active message (Router X is Router S), then Router X is also the peer-RPF neighbor, and its source-active messages are accepted.
2. If Router X is a member of the Router R mesh group, or is the configured peer, then Router X is the peer-RPF neighbor, and its source-active messages are accepted.
3. If Router X is the BGP next hop of the active multicast RPF route toward Router S (Router X installed the route on Router R), then Router X is the peer-RPF neighbor, and its source-active messages are accepted.
4. If Router X is an external BGP (EBGP) or internal BGP (IBGP) peer of Router R, and the last autonomous system (AS) number in the BGP AS-path to Router S is the same as Router X's AS number, then Router X is the peer-RPF neighbor, and its source-active messages are accepted.
5. If Router X uses the same next hop as the next hop to Router S, then Router X is the peer-RPF neighbor, and its source-active messages are accepted.
6. If Router X fits none of these criteria, then Router X is not an MSDP peer-RPF neighbor, and its source-active messages are rejected.

The MSDP peers that receive source-active TLVs can be constrained by BGP reachability information. If the AS path of the network layer reachability information (NLRI) contains the receiving peer's AS number prepended second to last, the sending peer is using the receiving peer as a next hop for this source. If the split horizon information is not being received, the peer can be pruned from the source-active TLV distribution list.