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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Building the PIM-SM multicast delivery tree<br />

The basic PIM-SM interaction with the RP is:<br />

1. A multicast router sends periodic join messages to a group-specific RP. Each<br />

router along the path toward the RP builds <strong>and</strong> sends join requests to the RP.<br />

This builds a group-specific multicast delivery tree rooted at the RP. Like<br />

other multicast protocols, the tree is actually a reverse path tree, because join<br />

requests follow a reverse path from the receiver to the RP. Figure 6-10 shows<br />

this function.<br />

Figure 6-10 Creating the RP-rooted delivery tree<br />

2. The multicast router connecting to the source initially encapsulates each<br />

multicast packet in a register message. These messages are sent to the RP.<br />

The RP decapsulates these unicast messages <strong>and</strong> forwards the data packets<br />

to the set of downstream receivers. Figure 6-11 shows this function.<br />

Figure 6-11 Registering a source<br />

264 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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