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This appendix provides the following information:

IGMP Overview

Supported Features and Functions

Detecting Multicast Routers

To configure the SmartSwitch device for IGMP operation, refer to 

IGMP is a multicast protocol used by routers. This protocol is supported 
by Cabletron Systems 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 request to 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. 

The following lists the features and functions supported when using 
IGMP:

Runs only when the switch is operating in the 802.1Q mode. 

Supports multiple multicast and non-multicast routers on the same 
VLAN.