diff --git a/openbsc/src/gsm_data.c b/openbsc/src/gsm_data.c
index 9db246c..a50ba8c 100644
--- a/openbsc/src/gsm_data.c
+++ b/openbsc/src/gsm_data.c
@@ -274,7 +274,7 @@
 			continue;
 		}
 
-		if (lac == 0 || bts->location_area_code == lac)
+		if (lac == GSM_LAC_RESERVED_ALL_BTS || bts->location_area_code == lac)
 			return bts;
 	}
 	return NULL;
diff --git a/openbsc/src/gsm_subscriber.c b/openbsc/src/gsm_subscriber.c
index a4e35c7..c81b522 100644
--- a/openbsc/src/gsm_subscriber.c
+++ b/openbsc/src/gsm_subscriber.c
@@ -105,7 +105,7 @@
 	case GSM_SUBSCRIBER_UPDATE_DETACHED:
 		/* Only detach if we are currently in this area */
 		if (bts->location_area_code == s->lac)
-			s->lac = 0;
+			s->lac = GSM_LAC_RESERVED_DETACHED;
 		dispatch_signal(SS_SUBSCR, S_SUBSCR_DETACHED, s);
 		break;
 	default:
diff --git a/openbsc/src/vty_interface.c b/openbsc/src/vty_interface.c
index 79124d9..d6f1bb5 100644
--- a/openbsc/src/vty_interface.c
+++ b/openbsc/src/vty_interface.c
@@ -872,6 +872,13 @@
 			lac, VTY_NEWLINE);
 		return CMD_WARNING;
 	}
+
+	if (lac == GSM_LAC_RESERVED_DETACHED || lac == GSM_LAC_RESERVED_ALL_BTS) {
+		vty_out(vty, "%% LAC %d is reserved by GSM 04.08%s",
+			lac, VTY_NEWLINE);
+		return CMD_WARNING;
+	}
+
 	bts->location_area_code = lac;
 
 	return CMD_SUCCESS;