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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 45 DF election
As shown in
, without the DF election mechanism, both Router B and Router C can receive
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:
1.
The router with a higher route preference becomes the DF.
2.
If the routers have the same route preference, the router with a lower metric becomes the DF.
3.
If the routers have the same metric, the router with a higher IP address becomes the DF.
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.
Ethernet
Router B
Router C
Router A
Multicast packets
DF election message
RP
Source
Router D
Router E
