Operation Manual – Multicast
H3C S3100 Series Ethernet Switches Chapter 2 IGMP Snooping Configuration
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Chapter 2 IGMP Snooping Configuration
2.1 IGMP Snooping Overview
Internet Group Management Protocol Snooping (IGMP Snooping) is a multicast
constraining mechanism that runs on Layer 2 devices to manage and control multicast
groups.
2.1.1 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
Figure 2-1, when IGMP Snooping is not running on the switch, multicast
packets are broadcast to all devices at Layer 2. 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. However, multicast packets for unknown
multicast groups are still broadcast at Layer 2.
Multicast packet transmission
without IGMP Snooping
Source
Multicast router
Host A
Receiver
Host B
Host C
Receiver
Multicast packets
Layer 2 switch
Multicast packet transmission
when IGMP Snooping runs
Source
Multicast router
Host A
Receiver
Host B
Host C
Receiver
Layer 2 switch
Figure 2-1 Before and after IGMP Snooping is enabled on Layer 2 device
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