H3C Technologies H3C S12500 Series Switches - Manual (Page 134)

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RP discovery
BIDIR-PIM uses the same RP discovery mechanism as PIM-SM does. For more information, see " RP
discovery ."
In PIM-SM, an RP must be specified with a real IP address. In BIDIR-PIM, however, an RP can be specified
with a virtual IP address, which is called the rendezvous point address (RPA). The link corresponding to
the RPA's subnet is called the rendezvous point link (RPL). All interfaces connected to the RPL can act as
the RP, and they back up one another.
NOTE:
In BIDIR-PIM, an RPF interface is the interface pointing to an RP, and an RPF neighbor is the address of the
next hop to the RP.
DF election
On a network segment with multiple multicast routers, duplicate multicast packets might be forwarded to
the RP. To address this issue, BIDIR-PIM uses a DF election mechanism to elect a unique designated
forwarder (DF) for each RP on every network segment within the BIDIR-PIM domain, and allows only the
DF to forward multicast data to the RP.
NOTE:
DF election is not necessary for an RPL.
Figure 43 DF election
As shown in Figure 43 , 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.
Ethernet
Router B Router C
Router A
Multicast packets
DF election message
RP
Source
Router D Router E