Multicast PIM Snooping

[lagapidis (Lazarus Agapidis)]

Hello Sam

@samirkhair

Also, I’m assuming this setting is only relevant if you have > 2 routers on the L2 segment as only then is there the possibility of ending up with a non-downstream DR. And in that situation, the (non-downstream) DR has identified the source as another multicast router which is why it does not send the PIM register message to the RP.

Exactly. You’ve got the right idea. the DR only generates PIM registers for locally sourced multicast, never for traffic it learns from another router.

The no ip pim snooping dr-flood command really comes into play on multi-router transit VLANs where the elected DR isn’t actually downstream for any receivers. In those cases, flooding toward the DR wastes bandwidth because it will just drop the traffic.

It’s true that the issue becomes most visible when there are >2 routers on the segment (otherwise the DR is almost always downstream anyway), but the logic is still the same: if the segment is just a transit link with no local multicast sources, disabling DR-flood avoids redundant traffic.

I hope this has been helpful!

Laz