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DF election

On a subnet with multiple multicast routers, duplicate multicast packets might be forwarded to the

RP. To address this issue, BIDIR-PIM uses a designated forwarder (DF) election mechanism to elect

a unique DF for each RP on a subnet. Only the DFs can forward multicast data to the RP.

DF election is not necessary for an RPL.

Figure 37 DF election

As shown in Figure 37, without the DF election mechanism, both Router B and Router C can receive

multicast packets from Route A. They also can 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

multicast a DF election message to all PIM routers (224.0.0.13) to initiate a DF election process. The

election message carries the RP's address, and the route preference and the metric of the unicast

route or static multicast route to the RP. A DF is elected as follows:

1. The router with a higher route preference becomes the DF.

2. If the routers have the same route preference, the router with a lower metric becomes the DF.

3. If the routers have the same metric, the router with a higher IP address becomes the DF.

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.

Ethernet

Router B Router C

Router A

Multicast packets

DF election message

Source

Router DRouter E

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