diff --git a/sys/net/if.c b/sys/net/if.c
--- a/sys/net/if.c
+++ b/sys/net/if.c
@@ -2853,12 +2853,8 @@
 		/*
 		 * If the link MTU changed, do network layer specific procedure.
 		 */
-		if (ifp->if_mtu != oldmtu) {
-#ifdef INET6
-			nd6_setmtu(ifp);
-#endif
-			rt_updatemtu(ifp);
-		}
+		if (ifp->if_mtu != oldmtu)
+			if_notifymtu(ifp);
 		break;
 	}
 
@@ -4470,6 +4466,15 @@
 	return (0);
 }
 
+void
+if_notifymtu(if_t ifp)
+{
+#ifdef INET6
+	nd6_setmtu(ifp);
+#endif
+	rt_updatemtu(ifp);
+}
+
 int
 if_getmtu(const if_t ifp)
 {
diff --git a/sys/net/if_bridge.c b/sys/net/if_bridge.c
--- a/sys/net/if_bridge.c
+++ b/sys/net/if_bridge.c
@@ -136,14 +136,6 @@
 
 #include <net/route.h>
 
-#ifdef INET6
-/*
- * XXX: declare here to avoid to include many inet6 related files..
- * should be more generalized?
- */
-extern void	nd6_setmtu(struct ifnet *);
-#endif
-
 /*
  * Size of the route hash table.  Must be a power of two.
  */
@@ -910,12 +902,8 @@
 		 * Bridge MTU may change during addition of the first port.
 		 * If it did, do network layer specific procedure.
 		 */
-		if (ifp->if_mtu != oldmtu) {
-#ifdef INET6
-			nd6_setmtu(ifp);
-#endif
-			rt_updatemtu(ifp);
-		}
+		if (ifp->if_mtu != oldmtu)
+			if_notifymtu(ifp);
 
 		if (bc->bc_flags & BC_F_COPYOUT)
 			error = copyout(&args, ifd->ifd_data, ifd->ifd_len);
diff --git a/sys/net/if_lagg.c b/sys/net/if_lagg.c
--- a/sys/net/if_lagg.c
+++ b/sys/net/if_lagg.c
@@ -77,14 +77,6 @@
 #include <net/if_lagg.h>
 #include <net/ieee8023ad_lacp.h>
 
-#ifdef INET6
-/*
- * XXX: declare here to avoid to include many inet6 related files..
- * should be more generalized?
- */
-extern void	nd6_setmtu(struct ifnet *);
-#endif
-
 #ifdef DEV_NETMAP
 MODULE_DEPEND(if_lagg, netmap, 1, 1, 1);
 #endif
@@ -1636,12 +1628,8 @@
 		 * LAGG MTU may change during addition of the first port.
 		 * If it did, do network layer specific procedure.
 		 */
-		if (ifp->if_mtu != oldmtu) {
-#ifdef INET6
-			nd6_setmtu(ifp);
-#endif
-			rt_updatemtu(ifp);
-		}
+		if (ifp->if_mtu != oldmtu)
+			if_notifymtu(ifp);
 
 		VLAN_CAPABILITIES(ifp);
 		break;
diff --git a/sys/net/if_var.h b/sys/net/if_var.h
--- a/sys/net/if_var.h
+++ b/sys/net/if_var.h
@@ -618,6 +618,7 @@
 int if_setmtu(if_t ifp, int mtu);
 int if_getmtu(const if_t ifp);
 int if_getmtu_family(const if_t ifp, int family);
+void if_notifymtu(if_t ifp);
 int if_setflagbits(if_t ifp, int set, int clear);
 int if_setflags(if_t ifp, int flags);
 int if_getflags(const if_t ifp);
diff --git a/sys/net/if_vlan.c b/sys/net/if_vlan.c
--- a/sys/net/if_vlan.c
+++ b/sys/net/if_vlan.c
@@ -85,14 +85,6 @@
 #include <netinet/if_ether.h>
 #endif
 
-#ifdef INET6
-/*
- * XXX: declare here to avoid to include many inet6 related files..
- * should be more generalized?
- */
-extern void	nd6_setmtu(struct ifnet *);
-#endif
-
 #define	VLAN_DEF_HWIDTH	4
 #define	VLAN_IFFLAGS	(IFF_BROADCAST | IFF_MULTICAST)
 
@@ -2110,12 +2102,8 @@
 		 * VLAN MTU may change during addition of the vlandev.
 		 * If it did, do network layer specific procedure.
 		 */
-		if (ifp->if_mtu != oldmtu) {
-#ifdef INET6
-			nd6_setmtu(ifp);
-#endif
-			rt_updatemtu(ifp);
-		}
+		if (ifp->if_mtu != oldmtu)
+			if_notifymtu(ifp);
 		break;
 
 	case SIOCGETVLAN: