--- a/src/flow-monitor/model/ipv4-flow-probe.cc	Sat May 14 04:50:07 2011 -0400
+++ b/src/flow-monitor/model/ipv4-flow-probe.cc	Sat May 14 04:52:55 2011 -0400
@@ -327,7 +327,9 @@
   bool tagFound;
   tagFound = ConstCast<Packet> (ipPayload)->RemovePacketTag (fTag);
   NS_ASSERT_MSG (tagFound, "FlowProbeTag is missing");
-
+  // cast tagFound to void, to suppress 'tagFound' set but not used compiler 
+  // warning in optimized builds
+  (void) tagFound;
   FlowId flowId = fTag.GetFlowId ();
   FlowPacketId packetId = fTag.GetPacketId ();
   uint32_t size = fTag.GetPacketSize ();