
Configuring IP Multicasting and Multimedia Services
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117355-D Rev 00
How a Receiver Leaves a Multicast Group
When a receiver wants to leave a multicast group, it sends an IGMP leave message
to the DR.
When all directly connected members of a multicast group leave the group or time
out and no downstream members remain, the DR sends a prune message upstream
and PIM deletes the route entry after that entry times out.
How a Source Sends Multicast Packets to a Group
The following steps, which are illustrated in Figure 6-4, describe how a source
sends multicast packets to a group:
1.
A source directly attached to a LAN sends a multicast data packet to the DR.
The DR (first-hop router) encapsulates each packet in a register message and
sends a unicast message directly to the RP for distribution down the shared
tree.
2.
The RP responds in the following ways if it has downstream group members
that want to receive multicast traffic:
• If the data rate of the source packet is within the data rate threshold
configured on the RP (the default is 1024 bytes per second), the RP
decapsulates each register message and forwards native multicast
messages downstream to all group members on the shared tree.
• If the data rate of the source packet exceeds the data rate threshold
configured on the RP, the RP can elect to switch to an SPT upon receiving
the first packet from the source.
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