diff --git a/net/api/src/main/java/org/onlab/onos/net/topology/ClusterId.java b/net/api/src/main/java/org/onlab/onos/net/topology/ClusterId.java
new file mode 100644
index 0000000..af8f122
--- /dev/null
+++ b/net/api/src/main/java/org/onlab/onos/net/topology/ClusterId.java
@@ -0,0 +1,49 @@
+package org.onlab.onos.net.topology;
+
+import java.util.Objects;
+
+import static com.google.common.base.MoreObjects.toStringHelper;
+
+/**
+ * Representation of the topology cluster identity.
+ */
+public final class ClusterId {
+
+    private final int id;
+
+    // Public construction is prohibit
+    private ClusterId(int id) {
+        this.id = id;
+    }
+
+    /**
+     * Returns the cluster identifier, represented by the specified integer
+     * serial number.
+     *
+     * @param id integer serial number
+     * @return cluster identifier
+     */
+    public static ClusterId clusterId(int id) {
+        return new ClusterId(id);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(id);
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (obj instanceof ClusterId) {
+            final ClusterId other = (ClusterId) obj;
+            return Objects.equals(this.id, other.id);
+        }
+        return false;
+    }
+
+    @Override
+    public String toString() {
+        return toStringHelper(this).add("id", id).toString();
+    }
+
+}
diff --git a/net/api/src/main/java/org/onlab/onos/net/topology/DefaultTopologyCluster.java b/net/api/src/main/java/org/onlab/onos/net/topology/DefaultTopologyCluster.java
new file mode 100644
index 0000000..f33dcf7
--- /dev/null
+++ b/net/api/src/main/java/org/onlab/onos/net/topology/DefaultTopologyCluster.java
@@ -0,0 +1,81 @@
+package org.onlab.onos.net.topology;
+
+import org.onlab.onos.net.DeviceId;
+
+import java.util.Objects;
+
+import static com.google.common.base.MoreObjects.toStringHelper;
+
+/**
+ * Default implementation of a network topology cluster.
+ */
+public class DefaultTopologyCluster implements TopologyCluster {
+
+    private final ClusterId id;
+    private final int deviceCount;
+    private final int linkCount;
+    private final DeviceId root;
+
+    /**
+     * Creates a new topology cluster descriptor with the specified attributes.
+     *
+     * @param id          cluster id
+     * @param deviceCount number of devices in the cluster
+     * @param linkCount   number of links in the cluster
+     * @param root        cluster root node
+     */
+    public DefaultTopologyCluster(ClusterId id, int deviceCount, int linkCount,
+                                  DeviceId root) {
+        this.id = id;
+        this.deviceCount = deviceCount;
+        this.linkCount = linkCount;
+        this.root = root;
+    }
+
+    @Override
+    public ClusterId id() {
+        return id;
+    }
+
+    @Override
+    public int deviceCount() {
+        return deviceCount;
+    }
+
+    @Override
+    public int linkCount() {
+        return linkCount;
+    }
+
+    @Override
+    public DeviceId root() {
+        return root;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(id, deviceCount, linkCount, root);
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (obj instanceof DefaultTopologyCluster) {
+            final DefaultTopologyCluster other = (DefaultTopologyCluster) obj;
+            return Objects.equals(this.id, other.id) &&
+                    Objects.equals(this.deviceCount, other.deviceCount) &&
+                    Objects.equals(this.linkCount, other.linkCount) &&
+                    Objects.equals(this.root, other.root);
+        }
+        return false;
+    }
+
+    @Override
+    public String toString() {
+        return toStringHelper(this)
+                .add("id", id)
+                .add("deviceCount", deviceCount)
+                .add("linkCount", linkCount)
+                .add("root", root)
+                .toString();
+    }
+}
diff --git a/net/api/src/main/java/org/onlab/onos/net/topology/Topology.java b/net/api/src/main/java/org/onlab/onos/net/topology/Topology.java
new file mode 100644
index 0000000..f71a5ec
--- /dev/null
+++ b/net/api/src/main/java/org/onlab/onos/net/topology/Topology.java
@@ -0,0 +1,50 @@
+package org.onlab.onos.net.topology;
+
+import org.onlab.onos.net.Provided;
+
+/**
+ * Represents a network topology computation snapshot.
+ */
+public interface Topology extends Provided {
+
+    /**
+     * Returns the time, specified in milliseconds since start of epoch,
+     * when the topology became active and made available.
+     *
+     * @return time in milliseconds since start of epoch
+     */
+    long time();
+
+    /**
+     * Returns the number of SCCs (strongly connected components) in the
+     * topology.
+     *
+     * @return number of clusters
+     */
+    int clusterCount();
+
+    /**
+     * Returns the number of infrastructure devices in the topology.
+     *
+     * @return number of devices
+     */
+    int deviceCount();
+
+
+    /**
+     * Returns the number of infrastructure links in the topology.
+     *
+     * @return number of links
+     */
+    int linkCount();
+
+    /**
+     * Returns the number of infrastructure paths computed between devices
+     * in the topology. This means the number of all the shortest paths
+     * (hop-count) between all device pairs.
+     *
+     * @return number of paths
+     */
+    int pathCount();
+
+}
diff --git a/net/api/src/main/java/org/onlab/onos/net/topology/TopologyCluster.java b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyCluster.java
index e58c784..46c9872 100644
--- a/net/api/src/main/java/org/onlab/onos/net/topology/TopologyCluster.java
+++ b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyCluster.java
@@ -1,10 +1,38 @@
 package org.onlab.onos.net.topology;
 
+import org.onlab.onos.net.DeviceId;
+
 /**
  * Representation of an SCC (strongly-connected component) in a network topology.
  */
 public interface TopologyCluster {
 
-    // TODO: add stuff in here: id, deviceCount, linkCount
+    /**
+     * Returns the cluster id.
+     *
+     * @return cluster identifier
+     */
+    ClusterId id();
+
+    /**
+     * Returns the number of devices in the cluster.
+     *
+     * @return number of cluster devices
+     */
+    int deviceCount();
+
+    /**
+     * Returns the number of infrastructure links in the cluster.
+     *
+     * @return number of cluster links
+     */
+    int linkCount();
+
+    /**
+     * Returns the device identifier of the cluster root device.
+     *
+     * @return cluster root device identifier
+     */
+    DeviceId root();
 
 }
diff --git a/net/api/src/main/java/org/onlab/onos/net/topology/TopologyDescription.java b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyDescription.java
index d9ec746..dd0102d 100644
--- a/net/api/src/main/java/org/onlab/onos/net/topology/TopologyDescription.java
+++ b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyDescription.java
@@ -1,8 +1,12 @@
 package org.onlab.onos.net.topology;
 
+import org.onlab.graph.Graph;
+import org.onlab.graph.GraphPathSearch;
 import org.onlab.onos.net.Description;
+import org.onlab.onos.net.DeviceId;
+import org.onlab.onos.net.Link;
 
-import java.util.Collection;
+import java.util.Set;
 
 /**
  * Describes attribute(s) of a network topology.
@@ -10,21 +14,58 @@
 public interface TopologyDescription extends Description {
 
     /**
-     * A collection of Device, Link, and Host descriptors that describe
-     * the changes tha have occurred in the network topology.
+     * Returns the creation timestamp of the topology description. This is
+     * expressed in system nanos to allow proper sequencing.
      *
-     * @return network element descriptions describing topology change
+     * @return topology description creation timestamp
      */
-    Collection<Description> details();
+    long timestamp();
 
-    // Default topology provider/computor should do the following:
-    // create graph
-    // search graph for SCC clusters (Tarjan)
-    // search graph for all pairs shortest paths based on hop-count
-    //      this means all shortest paths, between all pairs; not just one shortest path
-    // optionally use path results to produce destination-rooted broadcast trees
+    /**
+     * Returns the topology graph.
+     *
+     * @return network graph
+     */
+    Graph<TopoVertex, TopoEdge> graph();
 
-    // provide description with the graph, clusters, paths and trees upwards
+    /**
+     * Returns the results of the path search through the network graph. This
+     * is assumed to contain results of seach fro the given device to all
+     * other devices.
+     *
+     * @param srcDeviceId source device identifier
+     * @return path search result for the given source node
+     */
+    GraphPathSearch.Result pathResults(DeviceId srcDeviceId);
+
+    /**
+     * Returns the set of topology SCC clusters.
+     *
+     * @return set of SCC clusters
+     */
+    Set<TopologyCluster> clusters();
+
+    /**
+     * Returns the set of devices contained by the specified topology cluster.
+     *
+     * @return set of devices that belong to the specified cluster
+     */
+    Set<DeviceId> clusterDevices(TopologyCluster cluster);
+
+    /**
+     * Returns the set of infrastructure links contained by the specified cluster.
+     *
+     * @return set of links that form the given cluster
+     */
+    Set<Link> clusterLinks(TopologyCluster cluster);
+
+    /**
+     * Returns the topology SCC cluster which contains the given device.
+     *
+     * @param deviceId device identifier
+     * @return topology cluster that contains the specified device
+     */
+    TopologyCluster clusterFor(DeviceId deviceId);
 
 }
 
diff --git a/net/api/src/main/java/org/onlab/onos/net/topology/TopologyEvent.java b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyEvent.java
index fcbe858..0be5323 100644
--- a/net/api/src/main/java/org/onlab/onos/net/topology/TopologyEvent.java
+++ b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyEvent.java
@@ -1,7 +1,6 @@
 package org.onlab.onos.net.topology;
 
 import org.onlab.onos.event.AbstractEvent;
-import org.onlab.onos.net.Topology;
 
 /**
  * Describes network topology event.
diff --git a/net/api/src/main/java/org/onlab/onos/net/topology/TopologyProviderService.java b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyProviderService.java
index b3d85ad..0e03767 100644
--- a/net/api/src/main/java/org/onlab/onos/net/topology/TopologyProviderService.java
+++ b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyProviderService.java
@@ -1,7 +1,10 @@
 package org.onlab.onos.net.topology;
 
+import org.onlab.onos.event.Event;
 import org.onlab.onos.net.provider.ProviderService;
 
+import java.util.List;
+
 /**
  * Means for injecting topology information into the core.
  */
@@ -14,7 +17,9 @@
      * Signals the core that some aspect of the topology has changed.
      *
      * @param topoDescription information about topology
+     * @param reasons         events that triggered topology change
      */
-    void topologyChanged(TopologyDescription topoDescription);
+    void topologyChanged(TopologyDescription topoDescription,
+                         List<Event> reasons);
 
 }
diff --git a/net/api/src/main/java/org/onlab/onos/net/topology/TopologyService.java b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyService.java
index a6962a2..36ee666 100644
--- a/net/api/src/main/java/org/onlab/onos/net/topology/TopologyService.java
+++ b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyService.java
@@ -4,7 +4,6 @@
 import org.onlab.onos.net.ConnectPoint;
 import org.onlab.onos.net.DeviceId;
 import org.onlab.onos.net.Path;
-import org.onlab.onos.net.Topology;
 
 import java.util.Set;
 
@@ -37,8 +36,8 @@
     Graph<TopoVertex, TopoEdge> getGraph(Topology topology);
 
     /**
-     * Returns the set of all shortest paths, in terms of hop-count, between
-     * the specified source and destination devices.
+     * Returns the set of all shortest paths, precomputed in terms of hop-count,
+     * between the specified source and destination devices.
      *
      * @param topology topology descriptor
      * @param src      source device
@@ -71,7 +70,8 @@
 
 
     /**
-     * Indicates whether the specified connection point allows broadcast.
+     * Indicates whether the specified connection point belong to the
+     * broadcast tree.
      *
      * @param topology     topology descriptor
      * @param connectPoint connection point