diff --git a/src/bin/d2/d_cfg_mgr.h b/src/bin/d2/d_cfg_mgr.h
index c479f7eb5d..13e77a5e8e 100644
--- a/src/bin/d2/d_cfg_mgr.h
+++ b/src/bin/d2/d_cfg_mgr.h
@@ -1,4 +1,4 @@
-// Copyright (C) 2013 Internet Systems Consortium, Inc. ("ISC")
+// Copyright (C) 2013, 2015 Internet Systems Consortium, Inc. ("ISC")
 //
 // Permission to use, copy, modify, and/or distribute this software for any
 // purpose with or without fee is hereby granted, provided that the above
@@ -22,6 +22,13 @@
 #include <stdint.h>
 #include <string>
 
+// Undefine the macro OPTIONAL which is defined in some operating
+// systems but conflicts with class constant is the context base class.
+
+#ifdef OPTIONAL
+#undef OPTIONAL
+#endif
+
 namespace isc {
 namespace d2 {
 
diff --git a/src/bin/dhcp4/dhcp4_srv.h b/src/bin/dhcp4/dhcp4_srv.h
index 071f28e3fe..1a58279ea3 100644
--- a/src/bin/dhcp4/dhcp4_srv.h
+++ b/src/bin/dhcp4/dhcp4_srv.h
@@ -33,6 +33,14 @@
 #include <iostream>
 #include <queue>
 
+// Undefine the macro OPTIONAL which is defined in some operating
+// systems but conflicts with a member of the RequirementLevel enum in
+// the server class.
+
+#ifdef OPTIONAL
+#undef OPTIONAL
+#endif
+
 namespace isc {
 namespace dhcp {
 
diff --git a/src/bin/dhcp6/dhcp6_srv.h b/src/bin/dhcp6/dhcp6_srv.h
index 3dc32a461f..bdd4169e67 100644
--- a/src/bin/dhcp6/dhcp6_srv.h
+++ b/src/bin/dhcp6/dhcp6_srv.h
@@ -32,6 +32,14 @@
 #include <iostream>
 #include <queue>
 
+// Undefine the macro OPTIONAL which is defined in some operating
+// systems but conflicts with a member of the RequirementLevel enum in
+// the server class.
+
+#ifdef OPTIONAL
+#undef OPTIONAL
+#endif
+
 namespace isc {
 namespace dhcp {