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Cisco LAN Switching Fundamentals by Basir Sakandar - CCIE No. 6040, David Barnes - CCIE No. 6563

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IGMP Snooping

IGMP snooping prevents multicast flows from flooding to all ports on a VLAN by monitoring the Layer 3 IGMP packets. Multicast streams are sent to ports that explicitly request the flow. The switch via the IGMP snooping mechanism listens to the conversation between the router and the host machine. The switch learns at Layer 3 which port is signaling for or leaving a multicast group. The switching engine forwards this message to the Network Management Processor (NMP), where the port is added to or removed from the Layer 2 multicast forwarding group based on the IGMP message type.

The first step that is required for IGMP snooping is that the switch needs to learn the router port. Typically, a Protocol Independent Multicast (PIM) hello ...

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