This is a classic troubleshooting scenario in AOS-CX multicast environments. IGMP Snooping is designed to be " quiet " and efficient, but it has a specific dependency.

The Need for a Querier: IGMP Snooping works by listening for IGMP Join messages. However, for hosts to keep sending Join messages, there must be an IGMP Querier on the network that periodically asks, " Is anyone still listening to this multicast group? "

The Problem: If you enable IGMP Snooping on a VLAN but there is no L3 device acting as a Querier, the initial Join messages will eventually time out. The switch will then stop forwarding the multicast traffic to those ports because it assumes no one is listening anymore.

The Solution: By enabling IGMP on the Layer 3 interface (SVI) for that VLAN, the switch becomes the IGMP Querier for that subnet. It will send out periodic queries, the hosts will respond with " Reports, " and the IGMP Snooping table will remain populated, allowing the multicast application to continue functioning.

Why other options are incorrect:

Option A: IGMP Snooping is a Layer 2 function and is not configured " on " the L3 interface (SVI).

Options C and D: While PIM (Protocol Independent Multicast) is used for routing multicast between subnets, the immediate failure described (local applications not working on a VLAN after snooping is turned on) is almost always a lack of an IGMP Querier on that local segment.