In reply to UVM_beginner:
According to the UVM Coding Guidelines, do not use reset_phase.
Second, you should not use the uvm_config_db to set the default_sequence for any phase. Instead, you should specify the default sequence to be executed during the run_phase of your base test. Then you can use a derived test to execute any other sequences you want to execute during run_phase. You can also use the factory to change the default sequence either via an override in the derived test of via the command line.