How to find high fanout nets?
In Design Compiler you can use the command all_high_fanout -nets to obtain a collection of nets that has a fanout value bigger than high_fanout_net_threshold variable, which you can review using the command report_app_var high_fanout_net_threshold. A specific threshold N can also defined using the command:

all_high_fanout -net -threshold N


The reported high fanout nets are typically clock networks. If that is the case, you will see that the timing reports will have a huge increment in those nets, making timing closure much harder. Since the high-fanout issue is resolved during Clock Tree Synthesis of the Place &amp; Route phase those timing reports are too pessimistic. To overcome this you can define the clock networks as ideal by using the command set_ideal_network on the affected ports/pins.