Index: share/man/man4/lagg.4
===================================================================
--- share/man/man4/lagg.4
+++ share/man/man4/lagg.4
@@ -16,7 +16,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd November 18, 2017
+.Dd October 14, 2018
 .Dt LAGG 4
 .Os
 .Sh NAME
@@ -137,6 +137,9 @@
 .Pp
 The MTU of the first interface to be added is used as the lagg MTU.
 All additional interfaces are required to have exactly the same value.
+When changing the MTU, all ports must be changed to the new MTU, first.
+Care must be taken to avoid leaving interfaces with different MTU values
+when changing the MTU of ports.
 .Pp
 The
 .Ic loadbalance
Index: sys/net/if_lagg.c
===================================================================
--- sys/net/if_lagg.c
+++ sys/net/if_lagg.c
@@ -915,11 +915,6 @@
 		VLAN_CAPABILITIES(sc->sc_ifp);
 		break;
 
-	case SIOCSIFMTU:
-		/* Do not allow the MTU to be changed once joined */
-		error = EINVAL;
-		break;
-
 	default:
 		goto fallback;
 	}
@@ -1464,8 +1459,28 @@
 		break;
 
 	case SIOCSIFMTU:
-		/* Do not allow the MTU to be directly changed */
-		error = EINVAL;
+		if (ifr->ifr_mtu < 576) {
+			error = EINVAL;
+			break;
+		}
+		if (CK_SLIST_EMPTY(&sc->sc_ports)) {
+			sc->sc_ifp->if_mtu = ifr->ifr_mtu;
+			break;
+		}
+		LAGG_XLOCK(sc);
+		CK_SLIST_FOREACH(lp, &sc->sc_ports, lp_entries) {
+			if (lp->lp_ifp->if_mtu != ifr->ifr_mtu) {
+				if_printf(sc->sc_ifp,
+					  "invalid MTU: %u(%s) != %d\n",
+					  lp->lp_ifp->if_mtu,
+					  lp->lp_ifp->if_xname, ifr->ifr_mtu);
+				error = EINVAL;
+				break;
+			}
+		}
+		if (!error)
+			sc->sc_ifp->if_mtu = ifr->ifr_mtu;
+		LAGG_XUNLOCK(sc);
 		break;
 
 	default: