diff --git a/ryu/lib/packet/bpdu.py b/ryu/lib/packet/bpdu.py
index 8756518a..9834df79 100644
--- a/ryu/lib/packet/bpdu.py
+++ b/ryu/lib/packet/bpdu.py
@@ -180,9 +180,9 @@ class bpdu(packet_base.PacketBase):
         assert hasattr(self, 'VERSION_ID')
         assert hasattr(self, 'BPDU_TYPE')
 
-        self.protocol_id = PROTOCOL_IDENTIFIER
-        self.version_id = self.VERSION_ID
-        self.bpdu_type = self.BPDU_TYPE
+        self._protocol_id = PROTOCOL_IDENTIFIER
+        self._version_id = self.VERSION_ID
+        self._bpdu_type = self.BPDU_TYPE
 
         if hasattr(self, 'check_parameters'):
             self.check_parameters()
@@ -205,8 +205,8 @@ class bpdu(packet_base.PacketBase):
             return buf, None, None
 
     def serialize(self, payload, prev):
-        return struct.pack(bpdu._PACK_STR, self.protocol_id,
-                           self.version_id, self.bpdu_type)
+        return struct.pack(bpdu._PACK_STR, self._protocol_id,
+                           self._version_id, self._bpdu_type)
 
 
 @bpdu.register_bpdu_type
@@ -450,7 +450,7 @@ class RstBPDUs(ConfigurationBPDUs):
                  message_age=0, max_age=DEFAULT_MAX_AGE,
                  hello_time=DEFAULT_HELLO_TIME,
                  forward_delay=DEFAULT_FORWARD_DELAY):
-        self.version_1_length = VERSION_1_LENGTH
+        self._version_1_length = VERSION_1_LENGTH
 
         super(RstBPDUs, self).__init__(flags, root_priority,
                                        root_system_id_extension,
@@ -482,5 +482,5 @@ class RstBPDUs(ConfigurationBPDUs):
 
     def serialize(self, payload, prev):
         base = super(RstBPDUs, self).serialize(payload, prev)
-        sub = struct.pack(RstBPDUs._PACK_STR, self.version_1_length)
+        sub = struct.pack(RstBPDUs._PACK_STR, self._version_1_length)
         return base + sub
diff --git a/ryu/tests/unit/packet/test_packet.py b/ryu/tests/unit/packet/test_packet.py
index a9797774..b36a67eb 100644
--- a/ryu/tests/unit/packet/test_packet.py
+++ b/ryu/tests/unit/packet/test_packet.py
@@ -1441,9 +1441,9 @@ class TestPacket(unittest.TestCase):
 
         # bpdu
         ok_(p_bpdu)
-        eq_(bpdu.PROTOCOL_IDENTIFIER, p_bpdu.protocol_id)
-        eq_(bpdu.PROTOCOLVERSION_ID_BPDU, p_bpdu.version_id)
-        eq_(bpdu.TYPE_CONFIG_BPDU, p_bpdu.bpdu_type)
+        eq_(bpdu.PROTOCOL_IDENTIFIER, p_bpdu._protocol_id)
+        eq_(bpdu.PROTOCOLVERSION_ID_BPDU, p_bpdu._version_id)
+        eq_(bpdu.TYPE_CONFIG_BPDU, p_bpdu._bpdu_type)
         eq_(0, p_bpdu.flags)
         eq_(32768, p_bpdu.root_priority)
         eq_(0, p_bpdu.root_system_id_extension)
@@ -1484,10 +1484,7 @@ class TestPacket(unittest.TestCase):
                              if k in llc_values])
         llc_str = '%s(%s)' % (llc.llc.__name__, _llc_str)
 
-        bpdu_values = {'protocol_id': bpdu.PROTOCOL_IDENTIFIER,
-                       'version_id': bpdu.PROTOCOLVERSION_ID_BPDU,
-                       'bpdu_type': bpdu.TYPE_CONFIG_BPDU,
-                       'flags': 0,
+        bpdu_values = {'flags': 0,
                        'root_priority': long(32768),
                        'root_system_id_extension': long(0),
                        'root_mac_address': self.src_mac,