<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _rt_state_update(route_table_id, dcidr, router_ip="(none)", instance_id="(none)", eni_id="(none)", old_router_ip="(none)", msg="(none)"):
 """ Store a message about a VPC route in the current state. """

    buf = "inst: %s, eni: %s, r_ip: %-15s, o_r_ip: %-15s, msg: %s" % \
          (instance_id, eni_id, router_ip, old_router_ip, msg)
    CURRENT_STATE.vpc_state.setdefault('route_tables', {}). \
                            setdefault(route_table_id, {})[dcidr] = buf