IGMP Snooping is another Layer 2 function that helps to better manage the multicast traffic that a switch comes into contact with. Quite simply, not every environment is going to consist of Cisco equipment only. Routers and switches from other manufacturers may be present on the network, and these will be incapable of using the CGMP protocol. However, many vendors do implement a feature known as IGMP Snooping on their switches to help reconcile some of the multicast traffic issues mentioned earlier. A variety of Cisco Catalyst switch models do support IGMP Snooping, but it is important to recognize that at any given point in time, a Cisco switch can only be configured for either CGMP or IGMP Snooping – not both simultaneously.
As the name suggests, IGMP Snooping is a method that actually “snoops” or inspects IGMP traffic on a switch. When enabled, a switch will watch for IGMP messages passed between a host and a router, and will add the necessary ports to its multicast table, ensuring that only the ports that require a given multicast stream actually receive it. Unfortunately, IGMP Snooping suffers from one major drawback, namely the need for the switch to inspect all IGMP traffic, on top of its other responsibilities. However, in environments that do not support CGMP, IGMP Snooping provides a solid alternative to having all multicast traffic flooded to all ports.