If the number of routing tracks available in one particular area is less than the required number of routing tracks then it is called congestion.

Reasons for congestion:

• High standard cell density in an area.
• placement of standard cells near macros.
• high pin density at the edge of macro.
• bad floorplan.

How to fix congestion?

• Rerun the placement with congestion effort high.
• Modify physical constraints such as adjust cell density in congested areas.
• Use or modify proper blockages.
• Modify floorplanning to give enough room for routing.