An S5720HI supports
the stacking function since V200R009C00. When a member device in a
stack is faulty and fails to restart for three consecutive times,
the device attempts to roll back to a version earlier than V200R009C00
for restart. When the device restarts successfully after rolling back
to a version earlier than V200R009C00, a multi-active situation may
occur because the version earlier than V200R009C00 does not support
the stacking function. To prevent this situation, you are advised
to delete the system software earlier than V200R009C00 from member
devices when using S5720HIs to set up a stack.