Index: sys/net/if_lagg.c
===================================================================
--- sys/net/if_lagg.c
+++ sys/net/if_lagg.c
@@ -132,6 +132,7 @@
 static struct lagg_port *lagg_link_active(struct lagg_softc *,
 	    struct lagg_port *);
 static const void *lagg_gethdr(struct mbuf *, u_int, u_int, void *);
+static int	lagg_change_mtu(struct ifnet *ifp, struct ifreq *ifr);
 
 /* Simple round robin */
 static void	lagg_rr_attach(struct lagg_softc *);
@@ -1474,10 +1475,12 @@
 		break;
 
 	case SIOCSIFCAP:
-	case SIOCSIFMTU:
-		/* Do not allow the MTU or caps to be directly changed */
+		/* Do not allow the CAPs to be directly changed. */
 		error = EINVAL;
 		break;
+	case SIOCSIFMTU:
+		error = lagg_change_mtu(ifp, ifr);
+		break;
 
 	default:
 		error = ether_ioctl(ifp, cmd, data);
@@ -1755,6 +1758,73 @@
 	LAGG_WUNLOCK(sc);
 }
 
+static int
+lagg_change_mtu(struct ifnet *ifp, struct ifreq *ifr)
+{
+	struct lagg_softc *sc = (struct lagg_softc *)ifp->if_softc;
+	struct lagg_port *lp;
+	int old_mtu = ifp->if_mtu;
+	int new_mtu = ifr->ifr_mtu;
+	int err = 0, err2 = 0;
+	struct ifreq ifr_copy;
+
+	if (old_mtu == new_mtu)
+		return (0);
+
+	bcopy(ifr, &ifr_copy, sizeof(ifr_copy));
+
+	/* Try to change the MTU on all component interfaces; if that doesn't
+	 * work on any of them, roll back everything to the original value.
+	 */
+	LAGG_WLOCK(sc);
+	SLIST_FOREACH(lp, &sc->sc_ports, lp_entries) {
+		bcopy(lp->lp_ifp->if_xname, ifr_copy.ifr_name,
+		    IFNAMSIZ);
+		if (lp->lp_ioctl == NULL)
+			err = EOPNOTSUPP;
+		else
+			err = lp->lp_ioctl(lp->lp_ifp, SIOCSIFMTU,
+			    (caddr_t) &ifr_copy);
+		if (err) {
+			if_printf(lp->lp_ifp,
+			    "Failed to change MTU from %d to %d (err %d)\n",
+			    old_mtu, new_mtu, err);
+			/* Abort and roll-back after the first error. */
+			break;
+		}
+	}
+
+	/* No errors => all component interfaces' MTUs were changed. Update
+	 * the MTU of the LAGG as a whole.
+	 */
+	if (!err) {
+		ifp->if_mtu = new_mtu;
+		LAGG_WUNLOCK(sc);
+		return (0);
+	}
+
+	/* If we got here, then one of the component interfaces could not be
+	 * changed. Restore all component interfaces' MTUs to the original
+	 * value.
+	 */
+	ifr_copy.ifr_mtu = old_mtu;
+	SLIST_FOREACH(lp, &sc->sc_ports, lp_entries) {
+		bcopy(lp->lp_ifp->if_xname, ifr_copy.ifr_name,
+		    IFNAMSIZ);
+		if (lp->lp_ioctl == NULL)
+			err2 = EOPNOTSUPP;
+		else
+			err2 = lp->lp_ioctl(lp->lp_ifp, SIOCSIFMTU,
+			    (caddr_t) &ifr_copy);
+		if (err2)
+			if_printf(lp->lp_ifp,
+			    "Failed to restore MTU to %d (err %d)\n",
+			    old_mtu, err2);
+	}
+	LAGG_WUNLOCK(sc);
+	return (err);
+}
+
 struct lagg_port *
 lagg_link_active(struct lagg_softc *sc, struct lagg_port *lp)
 {
Index: sys/net/if_lagg.c
===================================================================
--- sys/net/if_lagg.c
+++ sys/net/if_lagg.c
@@ -132,6 +132,7 @@
 static struct lagg_port *lagg_link_active(struct lagg_softc *,
 	    struct lagg_port *);
 static const void *lagg_gethdr(struct mbuf *, u_int, u_int, void *);
+static int	lagg_change_mtu(struct ifnet *ifp, struct ifreq *ifr);
 
 /* Simple round robin */
 static void	lagg_rr_attach(struct lagg_softc *);
@@ -1474,10 +1475,12 @@
 		break;
 
 	case SIOCSIFCAP:
-	case SIOCSIFMTU:
-		/* Do not allow the MTU or caps to be directly changed */
+		/* Do not allow the CAPs to be directly changed. */
 		error = EINVAL;
 		break;
+	case SIOCSIFMTU:
+		error = lagg_change_mtu(ifp, ifr);
+		break;
 
 	default:
 		error = ether_ioctl(ifp, cmd, data);
@@ -1755,6 +1758,72 @@
 	LAGG_WUNLOCK(sc);
 }
 
+static int
+lagg_change_mtu(struct ifnet *ifp, struct ifreq *ifr)
+{
+	struct lagg_softc *sc = (struct lagg_softc *)ifp->if_softc;
+	struct lagg_port *lp;
+	int old_mtu = ifp->if_mtu;
+	int new_mtu = ifr->ifr_mtu;
+	int err = 0, err2 = 0;
+	struct ifreq ifr_copy;
+	struct rm_priotracker tracker;
+
+	if (old_mtu == new_mtu) {
+		return 0;
+	}
+
+	bcopy(ifr, &ifr_copy, sizeof(ifr_copy));
+
+	/* Try to change the MTU on all component interfaces; if that doesn't
+	 * work on any of them, for any reason, back everything out.
+	 */
+	LAGG_RLOCK(sc, &tracker);
+	SLIST_FOREACH(lp, &sc->sc_ports, lp_entries) {
+		if (lp->lp_ioctl == NULL) {
+			err = EOPNOTSUPP;
+		} else {
+			bcopy(lp->lp_ifp->if_xname, ifr_copy.ifr_name,
+			    IFNAMSIZ);
+			err = lp->lp_ioctl(lp->lp_ifp, SIOCSIFMTU,
+			    (caddr_t)&ifr_copy);
+		}
+		if (err) {
+			if_printf(lp->lp_ifp,
+			    "Failure changing MTU to %d (err %d)\n",
+			    new_mtu, err);
+			break;
+		}
+	}
+
+	if (!err) {
+		/* We are good */
+		ifp->if_mtu = new_mtu;
+		LAGG_RUNLOCK(sc, &tracker);
+		return 0;
+	}
+
+	/* restore mtu */
+	ifr_copy.ifr_mtu = old_mtu;
+	SLIST_FOREACH(lp, &sc->sc_ports, lp_entries) {
+		if (lp->lp_ioctl == NULL) {
+			continue;
+		} else {
+			bcopy(lp->lp_ifp->if_xname, ifr_copy.ifr_name,
+			    IFNAMSIZ);
+			err2 = lp->lp_ioctl(lp->lp_ifp, SIOCSIFMTU,
+			    (caddr_t)&ifr_copy);
+		}
+		if (err2) {
+			if_printf(lp->lp_ifp,
+			    "Failure restoring MTU to %d (err %d)\n",
+			    old_mtu, err2);
+		}
+	}
+	LAGG_RUNLOCK(sc, &tracker);
+	return err;
+}
+
 struct lagg_port *
 lagg_link_active(struct lagg_softc *sc, struct lagg_port *lp)
 {