Index: head/sys/net/if_lagg.c
===================================================================
--- head/sys/net/if_lagg.c
+++ head/sys/net/if_lagg.c
@@ -633,11 +633,18 @@
 {
 	struct lagg_softc *sc_ptr;
 	struct lagg_port *lp, *tlp;
-	int error, i;
+	struct ifreq ifr;
+	int error, i, oldmtu;
 	uint64_t *pval;
 
 	LAGG_XLOCK_ASSERT(sc);
 
+	if (sc->sc_ifp == ifp) {
+		if_printf(sc->sc_ifp,
+		    "cannot add a lagg to itself as a port\n");
+		return (EINVAL);
+	}
+
 	/* Limit the maximal number of lagg ports */
 	if (sc->sc_count >= LAGG_MAX_PORTS)
 		return (ENOSPC);
@@ -656,12 +663,25 @@
 		return (EPROTONOSUPPORT);
 
 	/* Allow the first Ethernet member to define the MTU */
-	if (CK_SLIST_EMPTY(&sc->sc_ports))
+	oldmtu = -1;
+	if (CK_SLIST_EMPTY(&sc->sc_ports)) {
 		sc->sc_ifp->if_mtu = ifp->if_mtu;
-	else if (sc->sc_ifp->if_mtu != ifp->if_mtu) {
-		if_printf(sc->sc_ifp, "invalid MTU for %s\n",
-		    ifp->if_xname);
-		return (EINVAL);
+	} else if (sc->sc_ifp->if_mtu != ifp->if_mtu) {
+		if (ifp->if_ioctl == NULL) {
+			if_printf(sc->sc_ifp, "cannot change MTU for %s\n",
+			    ifp->if_xname);
+			return (EINVAL);
+		}
+		oldmtu = ifp->if_mtu;
+		strlcpy(ifr.ifr_name, ifp->if_xname, sizeof(ifr.ifr_name));
+		ifr.ifr_mtu = sc->sc_ifp->if_mtu;
+		error = (*ifp->if_ioctl)(ifp, SIOCSIFMTU, (caddr_t)&ifr);
+		if (error != 0) {
+			if_printf(sc->sc_ifp, "invalid MTU for %s\n",
+			    ifp->if_xname);
+			return (error);
+		}
+		ifr.ifr_mtu = oldmtu;
 	}
 
 	lp = malloc(sizeof(struct lagg_port), M_DEVBUF, M_WAITOK|M_ZERO);
@@ -673,6 +693,9 @@
 		if (ifp == sc_ptr->sc_ifp) {
 			LAGG_LIST_UNLOCK();
 			free(lp, M_DEVBUF);
+			if (oldmtu != -1)
+				(*ifp->if_ioctl)(ifp, SIOCSIFMTU,
+				    (caddr_t)&ifr);
 			return (EINVAL);
 			/* XXX disable stacking for the moment, its untested */
 #ifdef LAGG_PORT_STACKING
@@ -681,6 +704,9 @@
 			    LAGG_MAX_STACKING) {
 				LAGG_LIST_UNLOCK();
 				free(lp, M_DEVBUF);
+				if (oldmtu != -1)
+					(*ifp->if_ioctl)(ifp, SIOCSIFMTU,
+					    (caddr_t)&ifr);
 				return (E2BIG);
 			}
 #endif
@@ -746,6 +772,8 @@
 	if ((error = lagg_proto_addport(sc, lp)) != 0) {
 		/* Remove the port, without calling pr_delport. */
 		lagg_port_destroy(lp, 0);
+		if (oldmtu != -1)
+			(*ifp->if_ioctl)(ifp, SIOCSIFMTU, (caddr_t)&ifr);
 		return (error);
 	}
 
@@ -1464,8 +1492,31 @@
 		break;
 
 	case SIOCSIFMTU:
-		/* Do not allow the MTU to be directly changed */
-		error = EINVAL;
+		LAGG_XLOCK(sc);
+		CK_SLIST_FOREACH(lp, &sc->sc_ports, lp_entries) {
+			if (lp->lp_ioctl != NULL)
+				error = (*lp->lp_ioctl)(lp->lp_ifp, cmd, data);
+			else
+				error = EINVAL;
+			if (error != 0) {
+				if_printf(ifp,
+				    "failed to change MTU to %d on port %s, "
+				    "reverting all ports to original MTU (%d)\n",
+				    ifr->ifr_mtu, lp->lp_ifp->if_xname, ifp->if_mtu);
+				break;
+			}
+		}
+		if (error == 0) {
+			ifp->if_mtu = ifr->ifr_mtu;
+		} else {
+			/* set every port back to the original MTU */
+			ifr->ifr_mtu = ifp->if_mtu;
+			CK_SLIST_FOREACH(lp, &sc->sc_ports, lp_entries) {
+				if (lp->lp_ioctl != NULL)
+					(*lp->lp_ioctl)(lp->lp_ifp, cmd, data);
+			}
+		}
+		LAGG_XUNLOCK(sc);
 		break;
 
 	default: