3Com corebuilder 3500 Implementation Manual page 345

Interface Relationships
The interface on which a router receives source-origin traffic for a given
source-group pair is called the incoming or parent interface. Each
interface over which the router forwards source-group traffic is called an
outgoing or child interface. A child interface on one router can:
Be a leaf interface — A subnetwork with group members
Lead to the parent interface of a downstream router — The next
router in the delivery path to reach group members
Broadcasting
The first packet for any source-group pair is broadcast across the entire
network, as far as packet time-to-live (TTL) and router TTL thresholds
permit. If a packet arrives on an interface that the router determines to be
the shortest path back to the source (by comparing interface metrics),
then the router forwards the packet on all interfaces except the incoming
interface. Downstream routers quickly send either:
Prune messages (explained next) to upstream routers if their interfaces
do not lead to group members
IGMP reports if they want to continue receiving traffic for that
source-group pair.
Some IP multicast applications try to actively send traffic on the network,
even if no group members are requesting their traffic. Your system can
detect which ports lead to routers and send these infrequent broadcast
packets only to those ports. Otherwise, the system filters all IP multicast
group traffic for which it has received no IGMP Reports or graft
messages.
Pruning
A parent interface transmits a prune message to its upstream neighboring
router if there are no group members on its child interfaces. A prune
message directs the upstream router not to forward packets for a
particular source-group pair in the future. Prune messages always affect
the entire routing interface; they cannot be targeted to prune individual
port segments that belong to an interface (IGMP snooping effectively
achieves this, however).
Prune messages always begin at the leaf routers and are sent one hop
back toward the source. Each successive router determines whether to
prune its connections.
How DVMRP Supports IP Multicast 345