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So just posting this hear as a word of warning. I'm seeing mld packets looped right back out the same interface the switch receives them on, violating a cardinal rule of switching, on ex4300mp's. This happens when the following 2 conditions are met.
1) the path to vstp root is on a port that is the non master RE.
2) mld-snooping is enabled on the vlan. We had the exact same thing happen with dhcp packets (v4) when the dhcp security was enabled on the vlan and path to root was on non master RE. This can be seen on case 2021-0222-0299. There is a PR that came from that case, PR1610253.
The behvavior is the exactly the same now, but with mld instead of dhcp. There is a jtac case open, 2023-0212-638031.
This is the 3rd time now that we've had issues with Junos devices not handling broadcast/flooded traffic correct. The problem seems to be endemic to the Junos platform.
Work around so far is to toggle RE or disable mld-snooping.