HP VPN Firewall Appliances Appendix Protocol Reference

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router in the PIM domain as the common node, or RP, through which the multicast data travels along
the RPT and reaches the receivers.
• When a receiver is interested in the multicast data addressed to a specific multicast group, the
router connected to this receiver sends a join message to the RP associated with that multicast group.
The path along which the message goes hop-by-hop to the RP forms a branch of the RPT.
• When a multicast source sends multicast streams to a multicast group, the source-side 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. After reaching the RP, the multicast packet is duplicated and delivered to the receivers
along the RPT.
Multicast traffic is duplicated only where the distribution tree branches, and this process automatically
repeats until the multicast traffic reaches the receivers.
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 shared-media network (such as Ethernet). The
elected DR will be the only multicast forwarder on this shared-media network.
A DR must be elected in a shared-media network, no matter this network connects to multicast sources or
to receivers. The receiver-side DR sends join messages to the RP. The source-side DR sends register
messages to the RP.
A DR is elected on a shared-media 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 shared-media network in a PIM-DM domain, a DR must
be elected to act as the IGMPv1 querier on that shared-media 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."