Introduction To Pim-Sm - H3C S7500 Series Operation Manual

Operation Manual – Multicast
H3C S7500 Series Ethernet Switches
V. Assert mechanism
In the shared network such as Ethernet, the same packets may be sent repeatedly. For
example, the LAN network segment contains many multicast routers, A, B, and C. They
each have their own receiving path to the multicast source S. As shown in
Figure 7-2 Diagram for assert mechanism
When Router A and Router B 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 C will receive two 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 messages. 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.

7.1.3 Introduction to PIM-SM

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
groups, 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
Router A
Ethernet
Assert message
Assert message
Multicast packets
Router B
Router C Receiver
7-4
Chapter 7 PIM Configuration
Figure 7-2: