3com 2928 Guida Utente
1-1
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.
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
mappings between ports and multicast MAC addresses and forwards multicast data based on these
mappings.
As shown in
, 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.
packets for known multicast groups are multicast to the receivers, rather than broadcast to all hosts, at
Layer 2.
Figure 1-1
Multicast forwarding before and after IGMP snooping runs
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
IGMP Snooping Related Ports
As shown in
, 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).