IGMP snooping configuration
Overview
Internet Group Management Protocol (IGMP) snooping is a multicast constraining mechanism that runs on Layer 2 devices to manage and control multicast groups.
Principle of IGMP snooping
By analyzing received IGMP messages, a Layer 2 device running IGMP snooping establishes mappings between ports and multicast MAC addresses and forwards multicast data based on these mappings.
As shown in a, when IGMP snooping is not running on the switch, multicast packets are flooded to all devices at Layer 2. However, when IGMP snooping is running on the switch, multicast packets for known multicast groups are multicast to the receivers, rather than broadcast to all hosts, at Layer 2.
a.Multicast forwarding before and after IGMP snooping runs
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| without IGMP Snooping |
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| when IGMP Snooping runs | |||||||||
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| Multicast router |
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| Multicast router |
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Source |
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| Layer 2 switch | Source |
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| Layer 2 switch | ||||||||
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| Host A |
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| Host C |
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Receiver |
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| Receiver | Receiver |
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| Receiver | ||||||||
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| Host B |
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| Multicast packets |
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IGMP snooping related ports
As shown in a, Router A connects to the multicast source, IGMP snooping runs on Switch A and Switch B, Host A and Host C are receiver hosts (namely, multicast group members).
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