Given that the PIM Join specifies the upstream neighbor, what is the purpose of a DR election on a broadcast segment?
Consider the topology below, where R1, R2 and R3 are on the same broadcast segment. An IGMP host is connected to R3
R3 sends a *,G Join towards the RP. This *,G Join contains an ‘upstream neighbor’ field which contains the address of its RPF neighbor R1. Though R2 sees this *,G Join, it does not forward the PIM Join upstream since its interface is not in the *,G Join’s ‘upstream neighbor’ field.
If this is true, why do we need the DR election between R1 and R2 to decide which of the two routers forwards the *,G Join? Isn’t that already decided by the ‘upstream neighbor’ field in the *,G Join?
The DR purpose /duty is to know which router on the multi-access segment will be the multicast querier ( the once that will process the igmp report and send the PIM join to the RP) you need a DR to avoid duplicate packets on the network, otherwise, all routers on the multi-access segment will send general queries to the igmp host and PIM joins to the RP in your example R3 will be the DR aka querier because the host is directly connected so he will always be the closes to the host, but if you put R3 at the same level as R1 and R2 and connect a switch where R3 is positioned it would make more sense.
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Lil DexxJNCIE-ENT#863, 3X JNCIP-[SP-ENT-DC], 4X JNCIA [cloud-DevOps-Junos-Design], Champions Ingenius, SSYB