[c-nsp] igmp helper-address

[R S]

Just to see if in this way the picture is seen better:

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Hi
I’ve three routers in a broadcast domain: A, B and C.

source ---- WAN ---- A ----- lan ----- C ---- receiver
                                         |
                                         B

A is the one where multicast traffic arrives from WAN (and also RP in my lab), B is the PIM DR of the lan and C is where igmp helper-address is configured (pim passive on the lan).

The only way to make the stub igmp helper configuration on router C is to make A as PIM DR.
I’m wondering to understand the reason ? 

Tks