IGMP snooping is used to monitor the traffic of IGMP protocol between hosts and multicast-enabled routers. The switch uses it to learn which interface has an interested receiver and forward multicast traffic only to these interfaces. This feature could save bandwidth by allowing it to send traffic only needed, rather than flooding all multicast packets to all interfaces.
To determine how to forward multicast traffic, IGMP snooping maintains a “table” about the interfaces information:
IGMP snooping learns1 about interfaces by monitor IGMP traffic. If an interface receives IGMP queries or PIM updates, it will be added to the table as a multicast-router interface. If an interface receives IGMP group membership reports, IGMP snooping adds it to the table as a group-member interface.
While both types are configurable by hand which is static, the dynamic learned information in the table about interfaces will age and be removed after missing updates.
Multicast traffic received on a switch interface with IGMP snooping is enabled is forwarded according to the following rules.
IGMP traffic is forwarded as follows:
Non-IGMP multicast traffic is forwarded as follows:
On my next post, I will show you some scenarios about IGMP snooping multicast forwarding.