Defining the Clock Trees

ICC uses the clock sources defined by the create_clock command as the clock roots and derives the default set of clock sinks by tracing through all cells in the transitive fanout of the clock roots.
You can designate either an input port or internal hierarchical pin as a clock source. To disallow clock sources defined on a hierarchical pin, set the cts_enable_clock_at_hierarchical_pin variable to false before using the create_clock command. This variable is true by default.

Note:
ICC will not synthesize a clock tree if its source is set to a constant value by using set_case_analysis.

In addtion to simple clock trees, ICC also supports cascaded clock trees ( a clock tree that containts  another clock tree in its fanout). The nested clock tree can either have its own source ( identified by the create_clock command) or be a generated clock ( identified by the create_generated_clock command).