Figure (1) Routing flow 
Routing is the process of creating physical connections based on logical connectivity. Signal pins are connected by routing metal interconnects. Routed metal paths must meet timing, clock skew, max trans/cap requirements and also physical DRC requirements.
In grid based routing system each metal layer has its own tracks and preferred routing direction which are defined in a unified cell in the standard cell library.
There are four steps of routing operations:
1. Global routing
2. Track assignment
3. Detail routing
4. Search and repair
Global Route assigns nets to specific metal layers and global routing cells. Global route tries to avoid congested global cells while minimizing detours. Global route also avoids pre-routed P/G, placement blockages and routing blockages.
Track Assignment (TA) assigns each net to a specific track and actual metal traces are laid down by it. It tries to make long, straight traces to avoid the number of vias. DRC is not followed in TA stage. TA operates on the entire design at once.
Detail Routing tries to fix all DRC violations after track assignment using a fixed size small area known as “SBox”. Detail route traverses the whole design box by box until entire routing pass is complete.
Search and Repair fixes remaining DRC violations through multiple iterative loops using progressively larger SBox sizes.
 Astro User Guide, Version X-2005.09, September 2005
- Physical Design Flow
- Inputs–outputs from physical design process
- Floor Planning
- Power Planning
- Timing Analysis in Physical Design
- Clock Tree Synthesis (CTS)