Juniper ACX1000 Configuration Manual page 734

ACX Series Universal Access Router Configuration Guide
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this message is actually called a join/prune message, but for clarity in this description, it
is called either join or prune, depending on its context.) The join message is multicast
hop by hop upstream to the ALL-PIM-ROUTERS group (224.0.0.13) by means of each
router's RPF interface until it reaches the RP. The RP router receives the (* ,G) PIM join
message and adds the interface on which it was received to the outgoing interface list
(OIL) of the rendezvous-point tree (RPT) forwarding state entry. This builds the RPT
connecting the receiver with the RP. The RPT remains in effect, even if no active sources
generate traffic.
NOTE: State—the (*,G) or (S,G) entries—is the information used for
forwarding unicast or multicast packets. S is the source IP address, G is the
multicast group address, and * represents any source sending to group G.
Routers keep track of the multicast forwarding state for the incoming and
outgoing interfaces for each group.
When a source becomes active, the source DR encapsulates multicast data packets into
a PIM register message and sends them by means of unicast to the RP router.
If the RP router has interested receivers in the PIM sparse-mode domain, it sends a PIM
join message toward the source to build a shortest-path tree (SPT) back to the source.
The source sends multicast packets out on the LAN, and the source DR encapsulates
the packets in a PIM register message and forwards the message toward the RP router
by means of unicast. The RP router receives PIM register messages back from the source,
and thus adds a new source to the distribution tree, keeping track of sources in a PIM
table. Once an RP router receives packets natively (with S,G), it sends a register stop
message to stop receiving the register messages by means of unicast.
In actual application, many receivers with multiple SPTs are involved in a multicast traffic
flow. To illustrate the process, we track the multicast traffic from the RP router to one
receiver. In such a case, the RP router begins sending multicast packets down the RPT
toward the receiver's DR for delivery to the interested receivers. When the receiver's DR
receives the first packet from the RPT, the DR sends a PIM join message toward the
source DR to start building an SPT back to the source. When the source DR receives the
PIM join message from the receiver's DR, it starts sending traffic down all SPTs. When
the first multicast packet is received by the receiver's DR, the receiver's DR sends a PIM
prune message to the RP router to stop duplicate packets from being sent through the
RPT. In turn, the RP router stops sending multicast packets to the receiver's DR, and
sends a PIM prune message for this source over the RPT toward the source DR to halt
multicast packet delivery to the RP router from that particular source.
If the RP router receives a PIM register message from an active source but has no
interested receivers in the PIM sparse-mode domain, it still adds the active source into
the PIM table. However, after adding the active source into the PIM table, the RP router
sends a register stop message. The RP router discovers the active source's existence and
no longer needs to receive advertisement of the source (which utilizes resources).
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