diff --git a/helper/ndn-global-routing-helper.cc b/helper/ndn-global-routing-helper.cc
index 6ecd944..04ecc6f 100644
--- a/helper/ndn-global-routing-helper.cc
+++ b/helper/ndn-global-routing-helper.cc
@@ -234,7 +234,7 @@
 }
 
 void
-GlobalRoutingHelper::CalculateRoutes ()
+GlobalRoutingHelper::CalculateRoutes (bool invalidatedRoutes/* = true*/)
 {
   /**
    * Implementation of route calculation is heavily based on Boost Graph Library
@@ -278,7 +278,10 @@
       // NS_LOG_DEBUG (predecessors.size () << ", " << distances.size ());
 
       Ptr<Fib>  fib  = source->GetObject<Fib> ();
-      fib->InvalidateAll ();
+      if (invalidatedRoutes)
+        {
+          fib->InvalidateAll ();
+        }
       NS_ASSERT (fib != 0);
 
       NS_LOG_DEBUG ("Reachability from Node: " << source->GetObject<Node> ()->GetId ());
@@ -324,7 +327,7 @@
 }
 
 void
-GlobalRoutingHelper::CalculateAllPossibleRoutes ()
+GlobalRoutingHelper::CalculateAllPossibleRoutes (bool invalidatedRoutes/* = true*/)
 {
   /**
    * Implementation of route calculation is heavily based on Boost Graph Library
@@ -350,7 +353,10 @@
 	}
 
       Ptr<Fib>  fib  = source->GetObject<Fib> ();
-      fib->InvalidateAll ();
+      if (invalidatedRoutes)
+        {
+          fib->InvalidateAll ();
+        }
       NS_ASSERT (fib != 0);
 
       NS_LOG_DEBUG ("===========");
diff --git a/helper/ndn-global-routing-helper.h b/helper/ndn-global-routing-helper.h
index 53c7b15..fa7dabd 100644
--- a/helper/ndn-global-routing-helper.h
+++ b/helper/ndn-global-routing-helper.h
@@ -97,19 +97,23 @@
 
   /**
    * @brief Calculate for every node shortest path trees and install routes to all prefix origins
+   *
+   * @param invalidatedRoutes flag indicating whether existing routes should be invalidated or keps as is
    */
   static void
-  CalculateRoutes ();
+  CalculateRoutes (bool invalidatedRoutes = true);
 
   /**
    * @brief Calculate all possible next-hop independent alternative routes
    *
+   * @param invalidatedRoutes flag indicating whether existing routes should be invalidated or keps as is
+   *
    * Refer to the implementation for more details.
    *
    * Note that this method is highly experimental and should be used with caution (very time consuming).
    */
   static void
-  CalculateAllPossibleRoutes ();
+  CalculateAllPossibleRoutes (bool invalidatedRoutes = true);
 
 private:
   void