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Neighbor discovery
BIDIR-PIM uses the same neighbor discovery mechanism as PIM-SM does. For more information, see
“Neighbor discovery.”
RP discovery
BIDIR-PIM uses the same RP discovery mechanism as PIM-SM does. For more information, see “RP
discovery.”
In PIM-SM, an RP must be specified with a real IP address. In BIDIR-PIM, however, an RP can be
specified with a virtual IP address, which is called the rendezvous point address (RPA). The link that
corresponds to the RPA’s subnet is called the “rendezvous point link (RPL)”. All interfaces connected to
the RPL can act as RPs, which back up one another.
NOTE:
In 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, BIDIR-PIM uses a DF election mechanism to elect a unique
designated forwarder (DF) for each RP on every network segment within the BIDIR-PIM domain, and
allows only the DF to forward multicast data to the RP.
NOTE:
DF election is not necessary for an RPL.
Figure 42 DF election
As shown in Figure 42, without the DF election mechanism, both Router B and Router C can receive
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:
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