Skewing of sink pins at post route

Hi , 

In my design, the clock tree looks good up to the routing stage, and I want the tool to preserve it as-is. However, during the post-route optimization phase using:

opt_design -post_route -report_dir reports -report_prefix postroute -setup -hold

I noticed that the tool inserts a significant number of USKC cells, which disrupts the clock path and introduces considerable skew, leading to numerous hold violations. My intention is to prevent the tool from modifying the clock tree or altering the skew at sink pins.

I attempted to preserve the clock tree after CTS by applying the following constraint:

set_db skew_group:<skew_group_name> .cts_skew_group_constrains all

Unfortunately, this approach didn’t work, and the post-route stage still modified the existing clock tree.