Index: os/lwip/src/core/ipv6/nd6.c
===================================================================
--- os/lwip/src/core/ipv6/nd6.c	(revision 8271)
+++ os/lwip/src/core/ipv6/nd6.c	(working copy)
@@ -462,7 +462,7 @@
         mtu_opt = (struct mtu_option *)buffer;
         if (htonl(mtu_opt->mtu) >= 1280) {
 #if LWIP_ND6_ALLOW_RA_UPDATES
-          inp->mtu = (u16_t)htonl(mtu_opt->mtu);
+          inp->mtu_ipv6 = (u16_t)htonl(mtu_opt->mtu);
 #endif /* LWIP_ND6_ALLOW_RA_UPDATES */
         }
         break;
@@ -1523,7 +1523,7 @@
       if (ip6_addr_islinklocal(ip6addr) ||
           nd6_is_prefix_in_netif(ip6addr, netif)) {
         /* Destination in local link. */
-        destination_cache[nd6_cached_destination_index].pmtu = netif->mtu;
+        destination_cache[nd6_cached_destination_index].pmtu = netif->mtu_ipv6;
         ip6_addr_copy(destination_cache[nd6_cached_destination_index].next_hop_addr, destination_cache[nd6_cached_destination_index].destination_addr);
       } else {
         /* We need to select a router. */
@@ -1788,7 +1788,7 @@
   }
 
   if (netif != NULL) {
-    return netif->mtu;
+    return netif->mtu_ipv6;
   }
 
   return 1280; /* Minimum MTU */
Index: os/lwip/src/include/lwip/netif.h
===================================================================
--- os/lwip/src/include/lwip/netif.h	(revision 8271)
+++ os/lwip/src/include/lwip/netif.h	(working copy)
@@ -261,6 +261,7 @@
 #endif /* LWIP_CHECKSUM_CTRL_PER_NETIF*/
   /** maximum transfer unit (in bytes) */
   u16_t mtu;
+  u16_t mtu_ipv6;
   /** number of bytes used in hwaddr */
   u8_t hwaddr_len;
   /** link level hardware address of this interface */