[ONOS-3857] BGP Update message builder.

Change-Id: I02d750f662602fc51b090e6beb89d73d5eb36436
diff --git a/protocols/bgp/bgpio/src/main/java/org/onosproject/bgpio/protocol/ver4/BgpFactoryVer4.java b/protocols/bgp/bgpio/src/main/java/org/onosproject/bgpio/protocol/ver4/BgpFactoryVer4.java
index c57832b..bf2ccdf 100755
--- a/protocols/bgp/bgpio/src/main/java/org/onosproject/bgpio/protocol/ver4/BgpFactoryVer4.java
+++ b/protocols/bgp/bgpio/src/main/java/org/onosproject/bgpio/protocol/ver4/BgpFactoryVer4.java
@@ -22,6 +22,7 @@
 import org.onosproject.bgpio.protocol.BgpMessageReader;
 import org.onosproject.bgpio.protocol.BgpNotificationMsg;
 import org.onosproject.bgpio.protocol.BgpOpenMsg;
+import org.onosproject.bgpio.protocol.BgpUpdateMsg;
 import org.onosproject.bgpio.protocol.BgpVersion;
 
 /**
@@ -47,6 +48,11 @@
     }
 
     @Override
+    public BgpUpdateMsg.Builder updateMessageBuilder() {
+        return new BgpUpdateMsgVer4.Builder();
+    }
+
+    @Override
     public BgpMessageReader<BgpMessage> getReader() {
         return BgpMessageVer4.READER;
     }