Construct GraphDescription with online/active Device/Link only

Change-Id: I9312c0c8ae190bf0200bd040823b6f7a3e7a15e1
diff --git a/core/net/src/main/java/org/onlab/onos/net/link/impl/LinkManager.java b/core/net/src/main/java/org/onlab/onos/net/link/impl/LinkManager.java
index 76ce64a..52b56c5 100644
--- a/core/net/src/main/java/org/onlab/onos/net/link/impl/LinkManager.java
+++ b/core/net/src/main/java/org/onlab/onos/net/link/impl/LinkManager.java
@@ -31,6 +31,7 @@
 import org.onlab.onos.net.ConnectPoint;
 import org.onlab.onos.net.DeviceId;
 import org.onlab.onos.net.Link;
+import org.onlab.onos.net.Link.State;
 import org.onlab.onos.net.MastershipRole;
 import org.onlab.onos.net.device.DeviceEvent;
 import org.onlab.onos.net.device.DeviceListener;
@@ -49,6 +50,8 @@
 import org.onlab.onos.net.provider.AbstractProviderService;
 import org.slf4j.Logger;
 
+import com.google.common.base.Predicate;
+import com.google.common.collect.FluentIterable;
 import com.google.common.collect.Sets;
 
 /**
@@ -109,6 +112,18 @@
     }
 
     @Override
+    public Iterable<Link> getActiveLinks() {
+        return FluentIterable.from(getLinks())
+                .filter(new Predicate<Link>() {
+
+                    @Override
+                    public boolean apply(Link input) {
+                        return input.state() == State.ACTIVE;
+                    }
+                });
+    }
+
+    @Override
     public Set<Link> getDeviceLinks(DeviceId deviceId) {
         checkNotNull(deviceId, DEVICE_ID_NULL);
         return Sets.union(store.getDeviceEgressLinks(deviceId),