code: def is_valid_mpls_label(label):
      
      if (not isinstance(label, numbers.Integral) or
              (4 <= label <= 15) or
              (label < 0 or label > 2 ** 20)):
          return False
  
      return True
docs: Validates `label` according to MPLS label rules
  
      RFC says:
      This 20-bit field.
      A value of 0 represents the "IPv4 Explicit NULL Label".
      A value of 1 represents the "Router Alert Label".
      A value of 2 represents the "IPv6 Explicit NULL Label".
      A value of 3 represents the "Implicit NULL Label".
      Values 4-15 are reserved.