What should I do if I do not want to route over the cell?

My design contained 4 cascaded nand2 gates, and I want to route only
with metal1 and metal2 layer with cells connected by abuttment.

In the soc encounter, I set CTSMode to topPreferredLayer to 2, and
bottomPreferredLayer to 1, then when routing, there are only metal1
and metal2, which is fine, but the metal1 routing still is on the top
of the cell, which is strange.


How can I solve this problem? Are there any settings that can be done
in the soc encounter, or should I make the whole cell area as blockage
when I generated .lef file from abstract generator?

Thanks in advance!!

Renee