3Com 7757 Configuration Manual page 464

464 C 47: PIM C
HAPTER ONFIGURATION
Introduction to PIM-SM
Assert mechanism
In the shared network such as Ethernet, the same packets may be sent repeatedly.
For example, the LAN network segments contains many multicast routers, A, B, C,
and D. They each have their own receiving path to the multicast source S. As
shown in Figure 113:
Figure 113 Diagram for assert mechanism
Router A
Ethernet
Assert message
Assert message
Multicast packets
When Router A, Router B, and Router C receive a multicast packet sent from the
multicast source S, they will all forward the multicast packet to the Ethernet. In
this case, the downstream node Router D will receive three copies of the same
multicast packet.
In order to avoid such cases, the Assert mechanism is needed to select one
forwarder. Routers in the network select the best path through sending Assert
packets. If two or more paths have the same priority and metric to the multicast
source, the router with the highest IP address will be the upstream neighbor of the
(S, G) entry, which is responsible for forwarding the (S, G) multicast packets. The
unselected routers will prune the corresponding interfaces to disable the
information forwarding.
Protocol independent multicast sparse mode (PIM-SM) is a sparse mode multicast
protocol. It is generally used in the following occasions where:
Group members are sparsely distributed
■
The range is wide
■
Large scaled networks
■
In PIM-SM, all hosts do not receive multicast packets by default. Multicast packets
are forwarded to the hosts which need multicast packets explicitly.
In order that the receiver can receive the multicast data streams of the specific
IGMP group, PIM-SM adopts rendezvous points (RP) to forward multicast
information to all PIM-SM routers with receivers. RP is adopted in multicast
forwarding. As a result, the network bandwidth that the data packets and control
packets occupy is reduced, and the processing overhead of the router is also
reduced.
Router B
Router C Receiver