What protocol is used by switches to listen to multicast messages between hosts and routers?

The correct answer is IGMP Snooping because it specifically refers to a mechanism used by network switches to monitor Internet Group Management Protocol (IGMP) traffic between hosts and routers. This allows switches to learn which devices are part of specific multicast groups. By listening to IGMP messages, switches can intelligently forward multicast traffic only to those ports that have devices interested in receiving it, thus optimizing bandwidth usage and improving overall network efficiency.

This process is crucial in managing multicast traffic effectively, preventing unnecessary transmission of multicast packets to all ports, which can lead to congestion and reduced network performance. By using IGMP Snooping, switches help ensure that multicast packets are delivered only to intended recipients, thereby conserving bandwidth and enhancing the quality of multicast services within the network.