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460 | Chapter 28. PIM
ProSafe M4100 and M7100 Managed Switches
PIM-SM
Protocol-independent multicast sparse mode (PIM-SM) is used to efficiently route multicast
traffic to multicast groups that can span wide area networks where bandwidth is a constraint.
Subnet 192.168.1.0/24
Source
Port
Port 1/0/9
Port
Subnet 192.168.3.0/24
IP 192.168.1.1
1/0/13
1/0/1
Subnet 192.168.6.0/24
Subnet 192.168.2.0/24
Subnet 192.168.5.0/24
Subnet 192.168.4.0/24
Host
IP 192.168.4.2
Port 1/0/10
Port
1/0/11
Port
1/0/21
Port 1/0/22
Switch A
Switch D
Switch B
Switch C
Port 1/0/22
Port
1/0/24
Port
1/0/21
Figure 46. PIM-SM
PIM-SM uses shared trees by default and implements source-based trees for efficiency; it
assumes that no hosts want the multicast traffic unless they specifically ask for it. It creates a
shared distribution tree centered on a defined rendezvous point (RP). Traffic from this source
is relayed to the receivers. Senders first send the multicast data to the RP, which in turn
sends the data down the shared tree to the receivers. Shared trees centered on an RP do not
necessarily provide the shortest, most optimal path. In such cases PIM-SM provides a means
to switch to more efficient source-specific trees. A data threshold rate is defined for toggling