HP MSR2003 Configuration Manual page 97

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 36 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:
1. Router B and Router C multicast a DF election message to all PIM routers (224.0.0.13). The
election message carries the RP's address, and the priority and metric of the unicast route, MBGP
route, or static multicast route to the RP.
2. The router with a route of the higher priority becomes the DF.
3. In the case of a tie in the priority, the router with the route with the lower metric wins the DF
election.
4. In the case of a tie in the metric, the router with the higher IP address wins.
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.
Router E
RP
Router B
Ethernet
DF election message
Multicast packets
Figure 36, without the DF election mechanism, both Router B and Router C can receive
Router D
Router C
Router A
Source
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