Juniper JUNOSE 11.1.X MULTICAST ROUTING, Configuration Manual

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Figure 9 on page 81 illustrates how PIM dense mode works. When a source sends
a multicast packet to a first-hop router, the first-hop router multicasts that packet to
its neighbors. Those neighbors in turn forward the packet to their neighbors and
their hosts that belong to the multicast group. If a neighbor has no hosts that belong
to the multicast group and has no other PIM neighbors, it returns a prune message
to the first-hop router. The first-hop router does not multicast subsequent packets
for that group to neighbors who respond with prune messages.
Figure 9: PIM Dense Mode Operation
Overriding Prunes
If a host on a previously pruned branch requests to join a multicast group, it sends
an IGMP message to its first-hop router. The first-hop router then sends a graft
message upstream.
PIM routers send join messages on multiaccess interfaces to override prune messages.
For example, if a PIM router sent a prune message to indicate that it had no hosts
for a multicast group, and one of its hosts subsequently requests to send a packet to
that group, the router sends a join message to the first-hop router.
Preventing Duplication
If there are parallel paths to a source, duplicate packets can travel downstream
through different routers to the network. If a forwarding router receives a multicast
packet on its outgoing interface, the router identifies that the packet is a duplicate
and notifies the upstream routers. See Figure 10 on page 82.
Overview
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Chapter 3: Configuring PIM for IPv4 Multicast