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IGMPv3 Join/Leave Messages, IGMP Filtering, and a 3750

mfarrenkopf
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Level 1

On a 3750 switch, IOS 12.2(25)SEE2 (also 12.2(53)SE1, to which we're slowly moving), IP Services feature set.  Routed Layer 3 access.

We're looking at implementing IGMP v3 in our standard configuration.  The documentation says that IGMP v3 join/leave messages are not supported when IGMP filtering is on.  What is the back story on this?  I understand IGMP filtering (allowing or disallowing certain multicast addresses or multicast address ranges or restricting the number of multicast groups as per http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_25_see/configuration/guide/swigmp.html#wp1117317).  But I don't understand why this impacts IGMPv3.  It doesn't seem like the two should be incompatible, so I'm at a loss to explain why it's not possible for the two to co-exist.  I also can't explain to my colleagues why having IGMP filtering on but unconfigured doesn't impact IGMPv1 or IGMPv2.  The 3750 defaults to IGMP filtering on but nothing configured.

We don't use IGMP filtering right now, so it's easy to shut off.  But I'd like to understand so that I could speak intelligently about it if we ever did feel a need for it.  I would welcome explanations, pointers to other documentation, other books, etc.

Thanks,

Matt

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Giuseppe Larosa
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