Configuring Multicast Source Discovery Protocol
How MSDP Works
IPC-478
Cisco IOS IP Configuration Guide
and the address or the originator ID of the RP, if configured. If the peer is an RP and has a member of
that multicast group, the data packet is decapsulated and forwarded down the shared-tree in the remote
domain.
The PIM designated router (DR) directly connected to the source sends the data encapsulated in a PIM
register message to the RP in the domain.
Note The DR sends the encapsulated data to the RP only once per source, when the source goes active. If
the source times out, this process happens again when it goes active again. This situation is different
from the periodic SA message that contains all sources that are registered to the originating RP. These
messages have no data.
Each MSDP peer receives and forwards the SA message away from the originating RP to achieve
peer-RPF flooding. The concept of peer-RPF flooding is with respect to forwarding SA messages. The
router 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 router forwards the message to all MSDP peers other than the RPF peer.
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 on to all its MSDP peers.
When an RP for a domain receives an SA message from an MSDP peer, it determines if it has any group
members interested in 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.
Figure78 MSDP Running Between RP Peers
MSDP peer
RP + MSDP peer
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Router B Receiver
MSDP peer
MSDP SA
MSDP SA
MSDP SA
TCP connection
BGP
Source
Multicast
Register
Peer RPF flooding
PIM sparse mode
domain
PIM
DR
(S, G) Join