Hi friends.
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
RP
/ \
/ \
R1 R2
| |
-----------
|
|
R3
|
IGMP host
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?
Thanks,
Deepak