2. Plane $\alpha // \beta, AB=6 \text{ cm}$ is a fixed line segment between $\alpha$ and $\beta$, $CD$ is a moving line segment between $\alpha$ and $\beta$, $CD \perp AB$. Then when the angle between $AB$ and $\beta$ is $60^{\circ}$, the minimum value of $CD$ is $\qquad$
$6 \sqrt{3} \text{ cm}$;