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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, IPv6 BIDIR-PIM uses a designated forwarder (DF) election mechanism to elect a unique
DF for each RP on each subnet in the IPv6 BIDIR-PIM domain. Only the DF can forward IPv6 multicast
data to the RP.
DF election is not necessary for an RPL.
Figure 90 DF election
As shown in Figure 90, without the DF election mechanism, both Router B and Router C can receive IPv6
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 IPv6 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 IPv6 PIM routers. 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 highest priority becomes the DF.
3. In the case of a tie in the priority, the router with the route with the lowest metric wins the DF
election.
4. In the case of a tie in the metric, the router with the highest link-local IPv6 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 IPv6 multicast sources as leaves. The
processes for building these two RPTs are different.
Ethernet
Router B Router C
Router A
IPv6 Multicast packets
DF election message
RP
Source
Router DRouter E
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