Cisco MDS 9000 Series Configuration Manual page 185

Congestion Detection, Avoidance, and Isolation
Congestion Isolation
Note The port monitor detects slow devices when a given rising-threshold is reached and triggers the congestion
isolation feature in the switch to move traffic to that slow device into the slow Virtual Link (VL2). The switch
does not automatically remove any devices from congestion isolation. This must be done manually once the
problem with the slow device is identified and resolved.
Cisco MDS 9000 Series Interfaces Configuration Guide, Release 8.x
167