Pim-Sm Operating Principle - 3Com 5500-SI Configuration Manual

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HAPTER ULTICAST ROTOCOL
PIM-SM Operating The working procedures for PIM-SM include: neighbor discovery, building the
Principle RP-rooted shared tree (RPT), multicast source registration and switch over to the SPT.
Neighbor Discovery
The PIM-SM router uses Hello messages to perform neighbor discovery when it is
started. All network nodes running PIM-SM stay in touch with one another by
periodically sending Hello messages.
Build the RP Shared Tree (RPT)
When hosts join a multicast group G, the leaf routers that directly connect with the
hosts send IGMP messages to learn the receivers of multicast group G. In this way, the
leaf routers calculate the corresponding rendezvous point (RP) for multicast group G
and then send join messages to the node of a higher level toward the rendezvous
point (RP). Each router along the path between the leaf routers and the RP will
generate (*, G) entries in the forwarding table, indicating that all packets sent to
multicast group G are applicable to the entries no matter from which source they are
sent. When the RP receives the packets sent to multicast group G, the packets will be
sent to leaf routers along the path built and then reach the hosts. In this way, an
RP-rooted tree (RPT) is built as shown in Figure 86.
Figure 86 RPT schematic diagram
RP
Multicast Source S
RPT
Receiver
join
Multicast source registration
Multicast Source Registration
When multicast source S sends a multicast packet to the multicast group G, the
PIM-SM multicast router directly connected to S will encapsulate the received packet
into a registration packet and send it to the corresponding RP in unicast form. If there
are multiple PIM-SM multicast routers on a network segment, the Designated Router
(DR) will be responsible for sending the multicast packet.