Index: sys/net/if_tun.c
===================================================================
--- sys/net/if_tun.c
+++ sys/net/if_tun.c
@@ -131,6 +131,15 @@
 
 static int	tun_clone_create(struct if_clone *, int, caddr_t);
 static void	tun_clone_destroy(struct ifnet *);
+
+#ifdef INET6
+/*
+ * XXX: declare here to avoid to include many inet6 related files..
+ * should be more generalized?
+ */
+extern void	nd6_setmtu(struct ifnet *);
+#endif
+
 static struct if_clone *tun_cloner;
 
 static d_open_t		tunopen;
@@ -665,13 +674,15 @@
 	int		error;
 	struct tun_softc *tp = dev->si_drv1;
 	struct tuninfo *tunp;
+	u_long oldmtu;
 
 	switch (cmd) {
 	case TUNSIFINFO:
 		tunp = (struct tuninfo *)data;
 		if (tunp->mtu < IF_MINMTU)
 			return (EINVAL);
-		if (TUN2IFP(tp)->if_mtu != tunp->mtu) {
+		oldmtu = TUN2IFP(tp)->if_mtu;
+		if (oldmtu != tunp->mtu) {
 			error = priv_check(td, PRIV_NET_SETIFMTU);
 			if (error)
 				return (error);
@@ -682,6 +693,19 @@
 		TUN2IFP(tp)->if_mtu = tunp->mtu;
 		TUN2IFP(tp)->if_baudrate = tunp->baudrate;
 		mtx_unlock(&tp->tun_mtx);
+		getmicrotime(&TUN2IFP(tp)->if_lastchange);
+		CURVNET_SET(TUN2IFP(tp)->if_vnet);
+		rt_ifmsg(TUN2IFP(tp));
+		/*
+		 * If the link MTU changed, do network layer specific procedure.
+		 */
+		if (TUN2IFP(tp)->if_mtu != oldmtu) {
+#ifdef INET6
+			nd6_setmtu(TUN2IFP(tp));
+#endif
+			rt_updatemtu(TUN2IFP(tp));
+		}
+		CURVNET_RESTORE();
 		break;
 	case TUNGIFINFO:
 		tunp = (struct tuninfo *)data;