H3C Technologies H3C S7500E Series Switches - Manual (Page 143)

6-6
How PIM-SM Works
The working mechanism of PIM-SM is summarized as follows:
z Neighbor discovery
z DR election
z RP discovery
z RPT building
z Multicast source registration
z Switchover to SPT
z Assert
Neighbor discovery
PIM-SM uses a similar neighbor discovery mechanism as PIM-DM does. For details, refer to
Neighbor discovery .
DR election
PIM-SM also uses hello messages to elect a DR for a multi-access network (such as Ethernet).
The elected DR will be the only multicast forwarder on this multi-access network.
A DR must be elected in a multi-access network, no matter this network connects to multicast
sources or to receivers. The DR at the receiver side sends join messages to the RP; the DR at
the multicast source side sends register messages to the RP.
z A DR is elected on a multi-access subnet by means of comparison of the priorities and IP
addresses carried in hello messages. An elected DR is substantially meaningful to PIM-SM.
PIM-DM itself does not require a DR. However, if IGMPv1 runs on any multi-access
network in a PIM-DM domain, a DR must be elected to act as the IGMPv1 querier on that
multi-access network.
z IGMP must be enabled on a device that acts as a receiver-side DR before receivers
attached to this device can join multicast groups through this DR.
For details about IGMP, refer to IGMP Configuration in the IP Multicast Configuration Guide.