diff --git a/src/etc/inc/interfaces.inc b/src/etc/inc/interfaces.inc
index 1818b8250..0fc368d09 100644
--- a/src/etc/inc/interfaces.inc
+++ b/src/etc/inc/interfaces.inc
@@ -2725,6 +2725,12 @@ function interface_6rd_configure($interface = 'wan', $wancfg)
     legacy_interface_create('stf', $stfiface);
     legacy_interface_flags($stfiface, 'link2');
 
+    $mtu = !empty($wancfg['mtu']) ? $wancfg['mtu'] - 20 : 1280;
+    if ($mtu > 1480) {
+        $mtu = 1480;
+    }
+    legacy_interface_mtu($stfiface, $mtu);
+
     mwexecf('/sbin/ifconfig %s inet6 %s/%s', array($stfiface, $rd6prefix, $rd6prefixlen + 32 - $wancfg['prefix-6rd-v4plen']));
     mwexecf('/sbin/ifconfig %s stfv4br %s', array($stfiface, $wancfg['gateway-6rd']));
     mwexecf('/sbin/ifconfig %s stfv4net %s/%s', array($stfiface, $ip4address, $wancfg['prefix-6rd-v4plen']));
@@ -2818,7 +2824,14 @@ function interface_6to4_configure($interface = 'wan', $wancfg)
     }
     legacy_interface_create('stf', $stfiface);
     legacy_interface_flags($stfiface, 'link2');
-    mwexec("/sbin/ifconfig {$stfiface} inet6 {$stflanpr} prefixlen 16");
+
+    $mtu = !empty($wancfg['mtu']) ? $wancfg['mtu'] - 20 : 1280;
+    if ($mtu > 1480) {
+        $mtu = 1480;
+    }
+    legacy_interface_mtu($stfiface, $mtu);
+
+    mwexecf('/sbin/ifconfig %s inet6 %s prefixlen 16', array($stfiface, $stflanpr));
 
     /* write out a default router file */
     file_put_contents("/tmp/{$wanif}_routerv6", "{$stfbrgw}");