Multicast DR

Unanswered Question
Jun 26th, 2009

Is it true that you only have a DR in a PIM-DM environment?

I have this problem too.
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Giuseppe Larosa Fri, 06/26/2009 - 09:51

Hello John,

as far as I know you have a PIM DR also on PIM SM as I've explained in the other thread

Hope to help


John Blakley Fri, 06/26/2009 - 09:57

Thanks Giuseppe. Maybe I should rephrase my question:

Does IGMPv2 use a DR? This book that I'm reading says that IGMPv2 is being used so the DR has no importance.

I read part of rfc 2236, and I can only find information about the IGMP Querier and non-querier, but nothing about a DR. Is that only used with IGMPv1?

Giuseppe Larosa Fri, 06/26/2009 - 10:28

Hello John,

IGMPv1 has no rules about IGMP querier so in a lan segment where IGMPv1 is in use the two

IGMP querier and PIM DR are the same device.

IGMPv2 realizing that it would be better to divide the workload on multiple routers if present use an opposite criteria for IGMP querier election:

lowest IP address on subnet

PIM DR uses highest ip address on subnet

(of the multicast capable routers)

Hope to help


Edison Ortiz Fri, 06/26/2009 - 10:52


As always great post. I want to clarify the IGMP querier election as this behavior may change depending upon the hardware that is used. For instance, on 6500 there isn't a querier election and you must make sure there is only one IGMP querier configured per Vlan.




Giuseppe Larosa Fri, 06/26/2009 - 11:05

Hello Edison,

thanks for your words

I see I was focusing on multicast enabled routers.

When we deal with IGMP snooping the IGMP snooping querier is an aid to allow to use IGMP snooping optimization on a vlan where we want to confine multicast (and so there is no multicast capable router running PIM) otherwise we should turn off IGMP snooping.

C6500 configuration guide advices to configure one and only one device as IGMP snooping querier.

Now I know why!

Hope to help



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