diff --git a/hdfs/CHANGES.txt b/hdfs/CHANGES.txt
index 25349d2227..efcf7b44b2 100644
--- a/hdfs/CHANGES.txt
+++ b/hdfs/CHANGES.txt
@@ -828,6 +828,9 @@ Trunk (unreleased changes)
 
     HDFS-2132. Potential resource leak in EditLogFileOutputStream.close. (atm)
 
+    HDFS-2120. on reconnect, DN can connect to NN even with different source
+    versions. (John George via atm)
+
 Release 0.22.0 - Unreleased
 
   INCOMPATIBLE CHANGES
diff --git a/hdfs/src/java/org/apache/hadoop/hdfs/server/common/IncorrectVersionException.java b/hdfs/src/java/org/apache/hadoop/hdfs/server/common/IncorrectVersionException.java
index ea82afb194..990b089fd7 100644
--- a/hdfs/src/java/org/apache/hadoop/hdfs/server/common/IncorrectVersionException.java
+++ b/hdfs/src/java/org/apache/hadoop/hdfs/server/common/IncorrectVersionException.java
@@ -45,4 +45,11 @@ public IncorrectVersionException(int versionReported,
           + versionReported + ". Expecting = " + versionExpected + ".");
   }
 
+  public IncorrectVersionException(String versionReported,
+                                   String ofWhat,
+                                   String versionExpected) {
+    super("Unexpected version " 
+          + (ofWhat==null ? "" : "of " + ofWhat) + ". Reported: "
+          + versionReported + ". Expecting = " + versionExpected + ".");
+  }
 }
diff --git a/hdfs/src/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java b/hdfs/src/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java
index 042d5e1729..9d476679d1 100644
--- a/hdfs/src/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java
+++ b/hdfs/src/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java
@@ -1143,7 +1143,26 @@ private void offerService() throws Exception {
     void register() throws IOException {
       LOG.info("in register: sid=" + bpRegistration.getStorageID() + ";SI="
           + bpRegistration.storageInfo); 
-                
+
+      // build and layout versions should match
+      String nsBuildVer = bpNamenode.versionRequest().getBuildVersion();
+      String stBuildVer = Storage.getBuildVersion();
+
+      if (!nsBuildVer.equals(stBuildVer)) {
+        LOG.warn("Data-node and name-node Build versions must be " +
+          "the same. Namenode build version: " + nsBuildVer + "Datanode " +
+          "build version: " + stBuildVer);
+        throw new IncorrectVersionException(nsBuildVer, "namenode", stBuildVer);
+      }
+
+      if (FSConstants.LAYOUT_VERSION != bpNSInfo.getLayoutVersion()) {
+        LOG.warn("Data-node and name-node layout versions must be " +
+          "the same. Expected: "+ FSConstants.LAYOUT_VERSION +
+          " actual "+ bpNSInfo.getLayoutVersion());
+        throw new IncorrectVersionException
+          (bpNSInfo.getLayoutVersion(), "namenode");
+      }
+
       while(shouldRun && shouldServiceRun) {
         try {
           // Use returned registration from namenode with updated machine name.
diff --git a/hdfs/src/test/hdfs/org/apache/hadoop/hdfs/server/datanode/TestDatanodeRegister.java b/hdfs/src/test/hdfs/org/apache/hadoop/hdfs/server/datanode/TestDatanodeRegister.java
new file mode 100644
index 0000000000..11e5f994fa
--- /dev/null
+++ b/hdfs/src/test/hdfs/org/apache/hadoop/hdfs/server/datanode/TestDatanodeRegister.java
@@ -0,0 +1,60 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hadoop.hdfs.server.datanode;
+
+import java.io.File;
+import java.io.IOException;
+import java.util.AbstractList;
+
+import static org.junit.Assert.fail;
+import static org.mockito.Mockito.*;
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.hdfs.server.common.IncorrectVersionException;
+import org.apache.hadoop.hdfs.server.protocol.DatanodeProtocol;
+import org.apache.hadoop.hdfs.server.protocol.DatanodeRegistration;
+import org.apache.hadoop.hdfs.server.protocol.NamespaceInfo;
+import org.junit.Test;
+
+
+public class TestDatanodeRegister { 
+  public static final Log LOG = LogFactory.getLog(TestDatanodeRegister.class);
+  @Test
+  public void testDataNodeRegister() throws Exception {
+    DataNode.BPOfferService myMockBPOS = mock(DataNode.BPOfferService.class);
+    doCallRealMethod().when(myMockBPOS).register();
+    myMockBPOS.bpRegistration = mock(DatanodeRegistration.class);
+    when(myMockBPOS.bpRegistration.getStorageID()).thenReturn("myTestStorageID");
+    
+    NamespaceInfo fakeNSInfo = mock(NamespaceInfo.class);
+    when(fakeNSInfo.getBuildVersion()).thenReturn("NSBuildVersion");
+    DatanodeProtocol fakeDNProt = mock(DatanodeProtocol.class);
+    when(fakeDNProt.versionRequest()).thenReturn(fakeNSInfo);
+    doCallRealMethod().when(myMockBPOS).setNameNode(fakeDNProt);
+    myMockBPOS.setNameNode( fakeDNProt );
+    try {   
+      myMockBPOS.register();
+      fail("register() did not throw exception! " +
+           "Expected: IncorrectVersionException");
+    } catch (IncorrectVersionException ie) {
+      LOG.info("register() returned correct Exception: IncorrectVersionException");
+    }
+  }
+}