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Local Management Supplement A-1
APPENDIX A
ABOUT IGMP
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
Section 1.19 for instructions.
A.1 IGMP OVERVIEW
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.
A.2 SUPPORTED FEATURES AND FUNCTIONS
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.