HP 6125XLG Ip Multicast Configuration Manual page 101

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 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.
Figure 39 RPT building at the receiver side
As shown in
in PIM-SM:
1. When a receiver wants to join the multicast group G, it uses an IGMP message to inform the
directly connected router.
2. After receiving the message, the router sends a join message, which is forwarded hop by hop to
the RP for the multicast group.
3. The routers along the path from the receiver's directly connected router to the RP form an RPT
branch. Each router on this branch adds a (*, G) entry to its forwarding table.
After a multicast receiver leaves the multicast group G, the directly connected router multicasts a prune
message to all 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. It also checks whether it still has
receivers for that multicast group. If not, the router continues to forward the prune message to its upstream
router.
Figure 39, the process for building a receiver-side RPT is similar to that for building an RPT
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