This paper presents a fast graph-based algorithm to evaluate routability of a floorplan. A simplified global routing problem is formulated as a constrained maximum independent set problem, and we use a limited number of routing options within a given number of routing layers. To test our approach, we use floorplans generated by the ELF-SP floorplanner that uses an evolutionary algorithm in conjunction with Lagrangian Relaxation. Experimental results show the effectiveness of our method.