Multicast routing is a networking method for efficient distribution of one-to-many traffic.
When your network has plenty of multicast traffic, the performance for the entire network can take a hit because of excessive false flooding.
IGMP (Internet Group Management Protocol) snooping optimizes that performance overhead.
The Router decides which devices will receive the applicable multicast traffic to improve the overall network speed.
Without IGMP Querying/Snooping, Multicast traffic is treated in the same manner as a Broadcast transmission, which forwards packets to all ports on the network.
IGMP Snooping generates no additional network traffic, which significantly reduces the Multicast traffic passing through your switch.
Enable IGMP when you frequently use any kind of streaming or mirroring from your devices to stream to IOT devices, such as Chromecast or Apple TV.
Do not turn it on unless actually needed. This will just add extra traffic, which the router will need to listen to and monitor; and could result in less performance.
Devices usually learn unicast MAC addresses by checking the source address field of the frames they receive and then send any traffic for that unicast address only to the appropriate interfaces.
A multicast MAC address, however, can never be the source address for a packet. As a result, when a device receives traffic for a multicast destination address, it floods the traffic on the relevant VLAN, sending a significant amount of traffic for which there might not necessarily be interested receivers.
IGMP snooping prevents this flooding.
When you enable IGMP snooping, the device monitors IGMP packets between receivers and multicast routers and uses the content of the packets to build a multicast forwarding table—a database of multicast groups and the interfaces that are connected to members of the groups.
When the device receives multicast packets, it uses the multicast forwarding table to selectively forward the traffic to only the interfaces that are connected to members of the appropriate multicast groups.
Multicast routers use IGMP to learn which groups have interested listeners for each of their attached physical networks.
In any given subnet, one multicast router acts as an IGMP querier. The IGMP querier sends out the following types of queries to hosts:
Hosts that are multicast listeners send the following kinds of messages:
Hosts can join multicast groups in two ways:
A multicast router continues to forward multicast traffic to a VLAN provided that at least one host on that VLAN responds to the periodic general IGMP queries.
Hosts can leave a multicast group in either of two ways: