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Chapter 25 Configuring MSDP
Understanding MSDP
The purpose of this topology is to have domains discover multicast sources in other domains. If the
multicast sources are of interest to a domain that has receivers, multicast data is delivered over the
normal, source-tree building mechanism in PIM-SM. MSDP is also used to announce sources sending
to a group. These announcements must originate at the domain RP.
MSDP depends heavily on the Border Gateway Protocol (BGP) or MBGP for interdomain operation. We
recommend that you run MSDP in RPs in your domain that are RPs for sources sending to global groups
to be announced to the Internet.
MSDP Operation
Figure 25-1
shows MSDP operating between two MSDP peers. PIM uses MSDP as the standard
mechanism to register a source with the RP of a domain. When MSDP is configured, this sequence
occurs.
When a source sends its first multicast packet, the first-hop router (designated router or RP) directly
connected to the source sends a PIM register message to the RP. The RP uses the register message to
register the active source and to forward the multicast packet down the shared tree in the local domain.
With MSDP configured, the RP also forwards a source-active (SA) message to all MSDP peers. The SA
message identifies the source, the group the source is sending to, and the address of the RP or the
originator ID (the IP address of the interface used as the RP address), if configured.
Each MSDP peer receives and forwards the SA message away from the originating RP to achieve
peer-RPF flooding. The MSDP device examines the BGP or MBGP routing table to determine which
peer is the next hop toward the originating RP of the SA message. Such a peer is called an RPF peer
(reverse-path forwarding peer). The MSDP device forwards the message to all MSDP peers other than
the RPF peer. For information on how to configure an MSDP peer when BGP and MBGP are not
supported, see the
“Configuring a Default MSDP Peer” section on page 25-4
.
If the MSDP peer receives the same SA message from a non-RPF peer toward the originating RP, it drops
the message. Otherwise, it forwards the message to all its MSDP peers.
When the RP for a domain receives the SA message from an MSDP peer, it determines if it has any join
requests for the group the SA message describes. If the (*,G) entry exists with a nonempty outgoing
interface list, the domain is interested in the group, and the RP triggers an (S,G) join toward the source.
After the (S,G) join reaches the source’s DR, a branch of the source tree has been built from the source
to the RP in the remote domain. Multicast traffic can now flow from the source across the source tree to
the RP and then down the shared tree in the remote domain to the receiver.
