128
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 subnet with multiple multicast routers, the same multicast packets might be forwarded to the
RP repeatedly. To address this issue, BIDIR-PIM uses a designated forwarder (DF) election
mechanism to elect a unique DF for each RP on each subnet in the BIDIR-PIM domain. Only the DF
can forward multicast data to the RP.
NOTE:
DF election is not necessary for an RPL.
Figure 46 DF election
As shown in
, without the DF election mechanism, Router B and Router C both can receive
multicast packets from Route A. They both might 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 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 directly connected to the receivers as leaves. The source-side
RPT is also rooted at the RP but takes the routers directly connected to the sources as leaves. The
processes for building these two parts are different.
Ethernet
Router B
Router C
Router A
Multicast packets
DF election message
RP
Source
Router D
Router E
