HP VPN Firewall Appliances Appendix Protocol Reference

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RPT is rooted at a router in the IPv6 PIM domain as the common node, or RP, through which the IPv6
multicast data travels along the RPT and reaches the receivers.
• When a receiver is interested in the IPv6 multicast data addressed to a specific IPv6 multicast group,
the router connected to this receiver sends a join message to the RP corresponding to that IPv6
multicast group. The path along which the message goes hop-by-hop to the RP forms a branch of the
RPT.
• When an IPv6 multicast source sends IPv6 multicast data to an IPv6 multicast group, the source-side
DR first registers the multicast source with the RP by unicasting register messages to the RP 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. The IPv6 multicast source sends subsequent IPv6 multicast packets along
the SPT to the RP. After reaching the RP, the IPv6 multicast packet is duplicated and delivered to the
receivers along the RPT.
IPv6 multicast traffic is duplicated only where the distribution tree branches, and this process
automatically repeats until the IPv6 multicast traffic reaches the receivers.
The working mechanism of IPv6 PIM-SM is summarized as follows:
• Neighbor discovery
• DR election
• RP discovery
• Embedded RP
• RPT building
• IPv6 multicast source registration
• Switchover to SPT
• Assert
Neighbor discovery
IPv6 PIM-SM uses the similar neighbor discovery mechanism as IPv6 PIM-DM does. For more information,
see "Neighbor discovery."
DR election
IPv6 PIM-SM also uses hello messages to elect a DR for a shared-media network (such as a LAN). The
elected DR will be the only IPv6 multicast forwarder on this shared-media network.
In the case of a shared-media network, a DR must be elected, no matter this network connects to IPv6
multicast sources or to receivers. The DR at the receiver side 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 comparing the priorities and IPv6 link-local addresses
carried in hello messages.
MLD must be enabled on a device that acts as a receiver-side DR. Otherwise, receivers attached to this
device cannot join IPv6 multicast groups through this DR. For more information about MLD, see "MLD."