Index: sys/netinet6/ip6_output.c
===================================================================
--- sys/netinet6/ip6_output.c
+++ sys/netinet6/ip6_output.c
@@ -1302,6 +1302,7 @@
 	uint32_t scopeid;
 	struct sockaddr_in6 *sa6_dst;
 	u_long mtu;
+	u_int32_t ifmtu;
 
 	mtu = 0;
 	if (do_lookup) {
@@ -1330,8 +1331,24 @@
 		mtu = ro_pmtu->ro_mtu;
 	}
 
-	if (ro_pmtu->ro_rt)
+	if (ro_pmtu->ro_rt) {
+		if (ifp == NULL)
+			ifp = ro_pmtu->ro_rt->rt_ifp;
+		ifmtu = IN6_LINKMTU(ifp);
 		mtu = ro_pmtu->ro_rt->rt_mtu;
+		/*
+		 * The MTU on the route is larger than the MTU on
+		 * the interface!  This shouldn't happen, unless the
+		 * MTU of the interface has been changed after the
+		 * interface was brought up.  Change the MTU in the
+		 * route to match the interface MTU (as long as the
+		 * field isn't locked).
+		 */
+		if (ifmtu < mtu) {
+			mtu = ifmtu;
+			ro_pmtu->ro_rt->rt_mtu = mtu;
+		}
+	}
 
 	return (ip6_calcmtu(ifp, dst, mtu, mtup, alwaysfragp, proto));
 }