Allied Telesis AT-SBx81CFC960 - Manual (Page 65)

SwitchBlade x8106 Chassis Switch and AT-SBx81CFC960 Card Installation Guide
Section I: Hardware Overview 65
One of the controller cards becomes the active master. In normal
operations, this is the controller card in slot 5. The active master manages
the system and processes CPU bound network traffic. The standby master
runs all network protocol modules and is kept in sync with the active
master card, so as to be available in hot-standby for near instantaneous
fail over, if required.
If the active master card is hot-swapped out, the standby master becomes
the active master. It takes over all control functions almost
instantaneously. Testing has shown no noticeable disruption to streaming
video.
You can determine the state of the controller card by viewing the CFC LED
on the card. The controller card is the active master when the LED is solid
green and the standby master when the LED is solid amber.