HP 6125XLG Ip Multicast Configuration Manual page 100

BIDIR-PIM addresses the problem. Derived from PIM-SM, BIDIR-PIM builds and maintains a bidirectional
RPT, which is rooted at the RP and connects the multicast sources and the receivers. Along the
bidirectional RPT, the multicast sources send multicast data to the RP, and the RP forwards the data to the
receivers. Each router along the bidirectional RPT needs to maintain only one (*, G) entry, saving system
resources.
BIDIR-PIM is suitable for a network with dense multicast sources and receivers.
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
discovery." In BIDIR-PIM, an RPF interface is the interface toward an RP, and an RPF neighbor is the
address of the next hop to the RP.
In PIM-SM, an RP must be specified with a real IP address. In BIDIR-PIM, an RP can be specified with a
virtual IP address, which is called the "rendezvous point address (RPA)." The link corresponding to the
RPA's subnet is called the "rendezvous point link (RPL)." All interfaces connected to the RPL can act as the
RPs, and they back up one another.
DF election
On a subnet with multiple multicast routers, duplicate multicast packets might be forwarded to the RP. To
address this issue, BIDIR-PIM uses a designated forwarder (DF) election mechanism to elect a unique DF
for each RP on each subnet in the BIDIR-PIM domain. Only the DF can forward multicast data to the RP.
DF election is not necessary for an RPL.
Figure 38 DF election
As shown in
multicast packets from Route A. They also can 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 E
RP
Router B
Ethernet
DF election message
Multicast packets
Figure 38, without the DF election mechanism, both Router B and Router C can receive
Router D
Router C
Router A
Source
90
"RP