H3C Technologies H3C S10500 Series Switches - Manual (Page 381)

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As shown in Figure 98 , without the DF election mechanism, both Router B and Router C can receive
multicast packets from Route A, and they might both 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 DF election messages to all PIM routers (224.0.0.13). The election
messages carry the RP’s address, and the priority and metric of the unicast route, MBGP route, or
multicast static route to the RP.
2. The router with a route of the highest priority becomes the DF.
3. In the case of a tie, 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 directly connected with the receivers as leaves. The source-side RPT is also
rooted at the RP but takes the routers directly connected with the IPv6 multicast sources as leaves. The
processes for building these two parts are different.
Figure 99 RPT building at the receiver side
As shown in Figure 99 , the process for building a receiver-side RPT is similar to that for building an RPT
in IPv6 PIM-SM:
1. When a receiver joins IPv6 multicast group G, it uses an MLD message to inform the directly
connected router.
2. After getting the receiver information, the router sends a join message, which is forwarded hop by
hop to the RP of the IPv6 multicast group.
3. The routers along the path from the receiver’s directly connected router to the RP form an RPT
branch, and each router on this branch adds a (*, G) entry to its forwarding table. The * means
any IPv6 multicast source.
When a receiver is no longer interested in the multicast data addressed to IPv6 multicast group G, the
directly connected router sends a prune message, which goes hop by hop along the reverse direction of