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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 123 RPT building at the receiver side
As shown in
, 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 asterisk
(*) means any IPv6 multicast source.
After a receiver host leaves the IPv6 multicast group G, the directly connected router multicasts a
prune message to all IPv6 PIM routers on the subnet. The prune message goes hop by hop along
the reverse direction of the RPT to the RP. After receiving the prune message, an upstream node
removes the interface that connects to the downstream node from the outgoing interface list. At the
same time, the upstream router checks the existence of receivers for that IPv6 multicast group. If no
receivers for the IPv6 multicast group exist, the router continues to forward the prune message to its
upstream router.
