Multicast Source Discovery Protocol (MSDP) is used to exchange multicast source information between BGP-enabled PIM-SM domains. Using MSDP, routers in a PIM-SM domain can rely on their own RP to reach a source in a different PIM-SM domain.

MSDP routers in a PIM-SM domain have a MSDP peering relationship with MSDP peers in another domain using a TCP connection. MSDP peering is the first step towards exchanging inter-domain multicast source information using MSDP SA (Source-Active) messages.

When an RP in a PIM-SM domain first learns of a new sender (via PIM register messages), it constructs an SA message and sends it to its MSDP peers.

All RPs which intend to originate or receive SA messages must establish MSDP peering with other RPs, either directly or via an intermediate MSDP peer.

An SA message contains these fields:

Each SA message received from a MSDP peer goes through an RPF check. The peer-RPF check compares the RP address carried in the SA message with the MSDP peer from which the message was received:

When an RP receives a new SA message from a peer in another domain, it checks if there are any receivers interested in the traffic. An RP checks for a (*, G) entry with a non-empty outgoing list. If the outgoing list is non-empty, the RP sends a (S,G) join towards the source.

If a member joins a group soon after a SA message is received by the local RP, that member needs to wait until the next SA message to learn about the source. MSDP SA caching is done at MSDP peers to reduce join latency for new receivers. The SA cache is populated as soon an MSDP peer receives a SA message from its peer.

MSDP Mesh groups are used inside a PIM-SM domain to ease RPF checking and SA forwarding within the domain. Any SA messages received from a peer in a mesh group are not forwarded to other peers in the same mesh group. This reduces SA message flooding and simplifies peer-RPF flooding.

An MSDP default peer is used when MSDP peers are not BGP peers. SA messages coming from a default peer do not go through an RPF check and are always accepted.

For the MSDP topology in Figure 7-11, you must enable PIM-SM on all the routers in both PIM domains and make RTR-1 a rendezvous point (RP) in Domain-1 and RTR-2 an RP in Domain-2. For the steps to configure PIM-SM and RPs, see Chapter 1, PIM Sparse Mode Configuration.

In the topology in Figure 7-11, an MSDP session is established between RTR-1 and RTR-2 in both domains. The following sample configuration on RTR-1 shows how to enable MSDP peering between RTR-1 and RTR-2.

IP addresses:

This command shows the MSDP peer information at RTR-1:

In the MSDP topology in Figure 7-11, when a source sends multicast traffic for group 224.1.1.1, RTR-4 (the DR) sends a register packet towards RTR-2 which is the RP in the domain. RTR-2 receives the register packet and sends an MSDP SA message to its MSDP peer (RTR-1). RTR-1 receives the SA message and creates an entry in the SA cache containing the source, group, and RP information.

This command at RTR-1 shows the SA information with source address, group address, and RP address:

RTR-3 receives an IGMP join for group 224.1.1.1 and joins the shared tree path toward the RP (RTR-1).

When RTR-1 receives an SA message from RTR-2, because it has a receiver, it sends an (S,G) join towards the source. Now traffic from the source is received at RTR-1 via the shortest path tree formed between RTR-1 and the source. RTR-1 distributes traffic downstream towards the receiver.

This command shows the PIM state at RTR-1 upon receiving an SA message and joining towards the source:

This command shows the MSDP peer information at RTR-2.