Multicast Snooping

IGMP Snooping is the process of listening to IGMP traffic. The available protocols are the Internet Group Membership Protocol (IGMP) for IPv4, and the Multicast Listener Discovery (MLD) protocol for IPv6.

When IGMP/MLD snooping is enabled in a switch on a VLAN, it analyses all of the IGMP/ MLD packets between hosts connected to the switch and Multicast routers in the network.

When a switch hears an IGMP/MLD report (join request) from a host for a given Multicast entry (this can be a group with any source or a group with a specific source), the switch adds the host's port number to the Multicast list for that entry.

IGMP/MLD snooping can effectively reduce Multicast traffic from streaming and other bandwidth-intensive IP applications. A switch using IGMP/MLD snooping only forwards Multicast traffic to the hosts interested in that traffic. This reduction of Multicast traffic reduces the packet processing at the switch and also reduces the workload at the end hosts since they do not have to receive and filter all of the Multicast traffic generated in the network.

The following are supported:

IGMP v1/v2/ v3
MLD v1/v2
A simple IGMP Snooping Querier

The IGMP Snooping Querier is a required IGMP device that facilitates the IGMP protocol and traffic flow on a given LAN. The IGMP Snooping Querier is used to support an L2 Multicast domain of snooping switches in the absence of a Multicast router. A typical example is a local network where the Multicast content is provided from a local server, and the router (if it exists at all) of that network does not support Multicast.

When the IGMP Snooping Querier is enabled, it starts after 60 seconds has passed with no IGMP traffic detected from a Multicast router. In the presence of other Multicast IGMP routers, the Querier stops sending queries on that VLAN, and begins sending them again after the router disappears.