Version 7.5.1.0 Introduced on the C-Series.
Version 6.5.1.0 Introduced on the E-Series.
Usage 
Information
By default, FTOS implements an optimized flush mechanism for RSTP. This helps in flushing 
MAC addresses only when necessary (and less often), allowing for faster convergence during 
topology changes. However, if a standards-based flush mechanism is needed, you can turn on 
this 
knob
 command to enable flushing MAC addresses after receiving every topology change 
notification.
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