Re-assembled the topology subsystem.
diff --git a/core/api/src/main/java/org/onlab/onos/net/topology/TopologyVertex.java b/core/api/src/main/java/org/onlab/onos/net/topology/TopologyVertex.java
new file mode 100644
index 0000000..96a918f
--- /dev/null
+++ b/core/api/src/main/java/org/onlab/onos/net/topology/TopologyVertex.java
@@ -0,0 +1,18 @@
+package org.onlab.onos.net.topology;
+
+import org.onlab.graph.Vertex;
+import org.onlab.onos.net.DeviceId;
+
+/**
+ * Represents a vertex in the topology graph.
+ */
+public interface TopologyVertex extends Vertex {
+
+ /**
+ * Returns the associated infrastructure device identification.
+ *
+ * @return device identifier
+ */
+ DeviceId deviceId();
+
+}