3Com 3C13636 Configuration Manual page 765

3Com Router 3000 Ethernet Family
Configuration Guide
data flows along the shared tree to the network segments where the multicast group
members are. When the data traffic is massive, the SPT (Shortest Path Tree) rooted on
the source can be used to reduce network delay.
PIM-SM depends on whichever present unicast routing table to perform the RPF check
instead of an unicast routing protocol.
I. PIM-SM Working Principle
The operation of PIM-SM includes neighbor discovery, Rendezvous point tree (RPT)
establishment, multicast source registration and SPT switching. Neighbor discovery
mechanism is the same with that of PIM-DM and will not be addressed here.
1) Rendezvous point tree (RPT) establishment
A leaf router locates the corresponding RP when it notices through IGMP packets that a
host directly connected to it joins in the multicast group G. It then sends the join
message to the upstream node towards the RP. Each router along the path between
the leaf router and the RP generates a (*, G) entry in the forwarding table, covering all
packets sent to the multicast group G, regardless of the sources. When receiving a
packet sent to multicast group G, the RP forwards it through the established path to the
leaf router and then to the host. In this way, an RP-rooted tree (RPT) is built as shown in
the following figure.
Multicast Source S Multicast Source S
Figure 4-2 RPT diagram
2) Multicast source registration
The multicast source S sends a multicast packet to the multicast group G. When the
PIM-SM multicast router directly connected to S receives the packet, it will encapsulate
the packet into a register 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 to send the multicast packet.
3) SPT switching
When a multicast router detects that the multicast packet with the destination address
of G from the RP is sent at a rate greater than the threshold, the multicast router will
RPT RPT
join join
Multicast source registration Multicast source registration
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4-3
Chapter 4 PIM Configuration
RP RP
Receiver Receiver