IGMP snooping is a Layer 2 mechanism that constrains multicast traffic at the VLAN level. By default, a Layer 2 switch floods multicast frames to all ports within a VLAN. With IGMP snooping enabled, the switch passively inspects (snoops) the IGMP control plane messages, specifically IGMP Membership Reports (Joins) and Leave messages, exchanged between hosts (receivers) and the multicast router. Based on this information, the switch builds a multicast forwarding database that maps multicast group addresses to the specific ports where interested receivers are connected. Consequently, it forwards multicast traffic only to these ports, effectively eliminating unnecessary flooding to ports without active listeners.

A. The switch populates its forwarding table with multicast group addresses and their associated ports, not the multicast source addresses.

B. This describes the behavior of an IGMP proxy, which is a distinct feature where the device actively participates in the IGMP protocol, rather than passively listening.

D. The switch inspects only the IGMP control messages to learn group memberships. Inspecting all multicast data traffic would be highly inefficient and is not how the mechanism works.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1, "IGMP Snooping" Section: "IGMP snooping is a feature that allows a switch to 'listen in' on the IGMP conversation between hosts and routers. When IGMP snooping is enabled on a switch, it examines all IGMP packets that are sent between hosts and multicast routers to determine which hosts are interested in receiving multicast traffic from a particular group. The switch then uses this information to build a map of which links need which multicast streams. Multicast traffic is then forwarded only to the links that have hosts that are interested in receiving the traffic." (Document ID: 3HE 17193 AAAD TQZZA, Part of the 7750 SR and 7950 XRS documentation suite).

2. RFC 4541, "Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches", Section 2.1.1: "A snooping switch must examine IGMP messages between hosts and routers to determine the multicast group membership of the hosts attached to its ports... Based on this information, the switch can then forward multicast traffic only to the ports that have interested receivers." (This RFC provides the foundational principles for IGMP snooping behavior).