3Com 5500-EI PWR Reference Manual page 400

Description
Use the unknown-multicast drop enable command to enable the function of dropping unknown
multicast packets.
Use the undo unknown-multicast drop enable command to disable the function of dropping unknown
multicast packets.
By default, the function of dropping unknown multicast packets is disabled.
Examples
Enable the unknown multicast drop feature.
<Sysname> system-view
System view: return to user view with Ctrl+Z.
[Sysname] unknown-multicast drop enable
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