HP FlexNetwork 7500 Series Configuration Manual page 111

Neighbor discovery
BIDIR-PIM uses the same neighbor discovery mechanism as PIM-SM does. For more information,
see "Neighbor
RP discovery
BIDIR-PIM uses the same RP discovery mechanism as PIM-SM does. For more information, see
"RP 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 a subnet. Only the DFs can forward multicast data to the RP.
DF election is not necessary for an RPL.
Figure 40 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
multicast a DF election message to all PIM routers (224.0.0.13) to initiate a DF election process. The
election message carries the RP's address, and the route preference and the metric of the unicast
route or static multicast route to the RP. A DF is elected as follows:
The router with a higher route preference becomes the DF.
1.
If the routers have the same route preference, the router with a lower metric becomes the DF.
2.
If the routers have the same metric, the router with a higher IP address becomes the DF.
3.
Bidirectional RPT building
A bidirectional RPT comprises a receiver-side RPT and a source-side RPT. The receiver-side RPT is
rooted at the RP and takes the routers that directly connect to the receivers as leaves. The
source-side RPT is also rooted at the RP but takes the routers that directly connect to the sources as
leaves. The processes for building these two RPTs are different.
discovery."
Router E
RP
Router B
Ethernet
DF election message
Multicast packets
Figure 40, without the DF election mechanism, both Router B and Router C can receive
Router D
Router C
Router A
Source
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