Internet Group Management Protocol (IGMP) is a multicast protocol used by routers. This
protocol is supported by Enterasys Networks SmartSwitches when operating in the 802.1Q mode
to “snoop” the IGMP frames. The multicast information is gleaned from the IGMP frame and a
filter is created to send the stream of data only to those end stations that will receive it.

IGMP queries are sent periodically from routers. An end station that will receive a multicast
stream will send a “query response” back to the router. If the router does not receive any response
from the end station, it stops forwarding the multicast streams to that station.

IGMP messages use an IP protocol number of 2. Routers send queries to the all-hosts group of
224.0.0.1. End stations send query responses to the multicast address of the stream they are
requesting to receive.

Routers need to be discovered and all multicast packets need to be sent to all routers.