Figure 9 on page 79 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 80.
79Copyright © 2010, Juniper Networks, Inc.
Chapter 3: Configuring PIM for IPv4 Multicast
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