The Multicast-to-Broadcast Fallacy

By default, an Ethernet switch is unaware of IP Multicast groups. When a multicast frame arrives, the switch sees a destination MAC address that isn't in its CAM table and, following standard logic, floods the frame out of every port in the VLAN. In a 48-port switch, this means 47 ports receive unwanted traffic.

How IGMP Snooping Intervenes

IGMP Snooping allows the switch to 'listen' (snoop) to the control plane exchange between hosts and routers. When a switch detects an IGMP Membership Report (Join), it notes the port and the specific multicast group address.

By maintaining a Layer 2 Multicast Table, the switch ensures that multicast streams are only delivered to ports that have explicitly requested them. This is the definition of Pruning.

The Querier and Membership Maintenance

To keep the membership table accurate, a Multicast Querier (usually the Layer 3 Gateway) must periodically send 'General Queries.' If a host fails to respond to a query, the switch times out the entry and stops forwarding that stream to the port.

• IGMPv2: Introduces the 'Leave Group' message, allowing for fast-leave processing.
• IGMPv3: Adds Source-Specific Multicast (SSM), allowing hosts to request traffic from a specific sender ($S,G$ instead of just $*,G$).

Engineering Challenges: Flooding and CPU

While efficient, IGMP Snooping is computationally expensive. The switch's CPU must inspect every IGMP packet (an 'Exception' to typical hardware-level forwarding). In large VLANs with hundreds of groups, this can lead to Control Plane Exhaustion.

Implementing IGMP Snooping is a non-negotiable requirement for modern networks carrying Video-over-IP, industrial control data, or high-fidelity audio streams. It is the bridge between Layer 3 intelligence and Layer 2 efficiency.