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I'm on an isolated (for now) catalyst 3560e. I have a multicast source, which when enabled, causes flooding to all switch interfaces. I have a second different multicast source, which does not cause flooding, it only gets delivered to the sinks who request it.

The second case leads me to believe the switch IGMP is configured properly, but what could be different about the first stream? I thought multicast/IGMP was 'I only get the stream if I ask for it'.

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3 Answers

Sounds like the difference between pim sparse mode and dense mode. Can you post the config you are using?

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The feature I believe you are referring to is IGMP snooping, in which a switch will keep track of which ports it has seen IGMP joins to specific groups.

What are the two groups in question? IGMP snooping is not applied to groups in the reserved range 224.0.0.0/24.

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up vote 1 down vote accepted

Thx for responses. Problem was default multicast packets had TTL=1 but the pim rendezvous point was a hop away thus never was discovered, so the switch flooded in attempt to find the RP. The working multicast had TTL=16.

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