diff --git a/apps/vtn/vtnrsc/src/main/java/org/onosproject/vtnrsc/RouterGateway.java b/apps/vtn/vtnrsc/src/main/java/org/onosproject/vtnrsc/RouterGateway.java
index d7868da..dd8911e 100644
--- a/apps/vtn/vtnrsc/src/main/java/org/onosproject/vtnrsc/RouterGateway.java
+++ b/apps/vtn/vtnrsc/src/main/java/org/onosproject/vtnrsc/RouterGateway.java
@@ -20,6 +20,7 @@
 
 import java.util.Collection;
 import java.util.Objects;
+import java.util.Set;
 
 /**
  * Representation of a Router gateway.
@@ -28,11 +29,11 @@
 
     private final TenantNetworkId networkId;
     private final boolean enableSnat;
-    private final Collection<FixedIp> externalFixedIps;
+    private final Set<FixedIp> externalFixedIps;
 
     // Public construction is prohibited
     private RouterGateway(TenantNetworkId networkId, boolean enableSnat,
-                         Collection<FixedIp> externalFixedIps) {
+                         Set<FixedIp> externalFixedIps) {
         this.networkId = checkNotNull(networkId, "networkId cannot be null");
         this.enableSnat = checkNotNull(enableSnat, "enableSnat cannot be null");
         this.externalFixedIps = checkNotNull(externalFixedIps, "externalFixedIps cannot be null");
@@ -47,7 +48,7 @@
      * @return RouterGateway
      */
     public static RouterGateway routerGateway(TenantNetworkId networkId, boolean enableSnat,
-                                              Collection<FixedIp> externalFixedIps) {
+                                              Set<FixedIp> externalFixedIps) {
         return new RouterGateway(networkId, enableSnat, externalFixedIps);
     }