--- linux-3.2.7/net/ipv4/route.c.orig	2012-02-28 17:26:00.214386221 +0100
+++ linux-3.2.7/net/ipv4/route.c	2012-02-28 17:25:27.826225630 +0100
@@ -1926,7 +1926,7 @@
 	const struct rtable *rt = (const struct rtable *) dst;
 	unsigned int mtu = dst_metric_raw(dst, RTAX_MTU);
 
-	if (mtu && rt_is_output_route(rt))
+	if (mtu && rt_is_output_route(rt) && mtu == dst->dev->mtu)
 		return mtu;
 
 	mtu = dst->dev->mtu;
@@ -1940,6 +1940,8 @@
 	if (mtu > IP_MAX_MTU)
 		mtu = IP_MAX_MTU;
 
+	dst_metric_set(dst, RTAX_MTU, mtu);
+
 	return mtu;
 }