Allied Telesis AT-S99 - Manual (Page 286)

Chapter 11: Uploading and Downloading Files
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During this process, which takes about two minutes to complete, the
management card will be unresponsive to commands.
6. To resume managing the chassis, start a new management session.
7. If the chassis has two management cards, reinstall the standby card.
After the standby card has initialized its management software, the
active management card automatically sends the new master
configuration file to the standby card over the backplane of the
chassis.