Router Initial Congestion Reporting - 2025.2 English - UG906

The Router Initial Congestion report (called Initial Estimated Router Congestion for 7 series FPGAs) is available only after the router has been run. It shows the routing congestion encountered by the router during the early stages of routing.

When the congestion level is 5 or higher, the report_design_analysis command generates a congestion table with details about the nature of the congestion and the region(s) associated with the highest congestion in a particular direction and type:

Global congestion

Estimated similar to placer congestion and based on all types of interconnects.

Long congestion

Considers only long interconnect utilization for a given direction.

Short congestion

Considers all other interconnect utilization for a given direction.

Any congestion area greater than 32×32 (level 5) can impact QoR and routability. Congestion on long interconnects increases short interconnect usage, leading to longer routed delays. Congestion on short interconnects can increase runtimes and, when widespread, degrade QoR. To analyze, follow these steps:

1. Open the Router Initial Congestion table.
2. If the congestion level is greater than 6, the design is unlikely to meet timing and might fail during routing.
3. If the congestion level is 4 or 5, identify the module(s) located in the congested area(s).
4. Apply a congestion alleviation technique to these modules or rerun placement with different directives, such as SpreadLogic.
5. If the congestion level is 3 or less, congestion is probably not a cause for concern unless the design has a very tight timing budget.

The previous figure shows an example where regions with congestion level 5 or higher are reported. To generate a congestion report with a lower congestion threshold, use the -min_congestion_level switch (default is 5, valid range 3–8).

In addition to the region with the maximum congestion in a given direction and type, the report can also contain other regions with the same maximum congestion level in that direction and type. These regions might overlap or appear in different areas of the device.

The following figure shows an example where the design has a congestion level 6 for North (Direction) Long (Type) in more than one region.