HP A6600 Configuration Manual page 317

Neighbor discovery
IPv6 BIDIR-PIM uses the same neighbor discovery mechanism as IPv6 PIM-SM does. For more information,
see "Neighbor
RP discovery
IPv6 BIDIR-PIM uses the same RP discovery mechanism as IPv6 PIM-SM does. For more information, see
"RP discovery."
In IPv6 PIM-SM, an RP must be specified with a real IPv6 address. In IPv6 BIDIR-PIM, however, an RP can
be specified with a virtual IPv6 address, which is called the RPA. The link corresponding to the RPA
subnet is called RPL. All interfaces connected to the RPL can act as RPs, which back up one another.
In IPv6 BIDIR-PIM, an RPF interface is the interface pointing to an RP, and an RPF neighbor is the address
of the next hop to the RP.
DF election
On a network segment with multiple multicast routers, the same multicast packets might be forwarded to
the RP repeatedly. To address this issue, IPv6 BIDIR-PIM uses a DF election mechanism to elect a unique
designated forwarder (DF) for each RP on every network segment within the IPv6 BIDIR-PIM domain, and
allows only the DF to forward multicast data to the RP.
DF election is not necessary for an RPL.
Figure 91 DF election
As shown in
multicast packets from Route A, and they might both forward the packets to downstream routers on the
local subnet. As a result, the RP (Router E) receives duplicate multicast packets. With the DF election
mechanism, once receiving the RP information, Router B and Router C initiate a DF election process for
the RP:
Router B and Router C multicast DF election messages to all PIM routers (224.0.0.13). The election
1.
messages carry the RP's address, and the priority and metric of the unicast route, MBGP route, or
multicast static route to the RP.
The router with a route of the highest priority becomes the DF.
2.
In the case of a tie, the router with the route with the lowest metric wins the DF election.
3.
In the case of a tie in the metric, the router with the highest link-local IPv6 address wins.
4.
discovery."
Figure 91, without the DF election mechanism, both Router B and Router C can receive
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