H3C Technologies H3C SecPath F1000-E - Manual (Page 373)

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• When a multicast source sends multicast streams to a multicast group, the source-side designated
router (DR) first registers the multicast source with the RP by sending register messages to the RP by
unicast until it receives a register-stop message from the RP. The arrival of a register message at the
RP triggers the establishment of an SPT. Then, the multicast source sends subsequent multicast
packets along the SPT to the RP. Upon reaching the RP, the multicast packet is duplicated and
delivered to the receivers along the RPT.
NOTE:
Multicast traffic is duplicated only where the distribution tree branches, and this process
automatically repeats until the multicast traffic reaches the receivers.
How PIM-SM Works
The working mechanism of PIM-SM is summarized as follows:
• Neighbor discovery
• DR election
• RP discovery
• RPT building
• Multicast source registration
• Switchover to SPT
• Assert
Neighbor discovery
PIM-SM uses a similar neighbor discovery mechanism as PIM-DM does. For more information, see
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.
NOTE:
•
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.
• 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 more information about IGMP, see IGMP Configuration in the Firewall Web Configuration
Manual.