In a standard Layer 2 switch environment without specific multicast optimizations, multicast traffic is treated similarly to broadcast traffic and isflooded out of all portswithin a VLAN (except the port where it was received). This is inefficient as it consumes unnecessary bandwidth on ports where no receivers are present.

To optimize this,IGMP Snoopingis the standard solution.

How it works:The switch "snoops" or monitors IGMP (Internet Group Management Protocol) messages between hosts and routers.

Result:When a receiver sends an IGMP Membership Report (Join) for a specific multicast group, the switch records which port that request came from. It then builds a Layer 2 multicast forwarding table so that traffic for that group isonly forwardedto the specific ports that have active listeners.

Junos OS 24.4 Configuration:On modern Juniper switches, you enable this globally or per VLAN using the set protocols igmp-snooping vlan command.

Option A (Promiscuous mode)is used for monitoring or specific security configurations (like Private VLANs) and would not limit multicast flooding.Option B (Spanning Tree)manages loop prevention and does not differentiate between traffic types for forwarding optimization.Option D (IRB interface)is used for Layer 3 routing between VLANs but does not inherently provide Layer 2 multicast optimization within a single VLAN.