6lowpan nd: prefer NCEs over FIB entries

If an address can be found in the neighbor cache, it should be used - whatever the FIB may return.

sys/net/gnrc/network_layer/sixlowpan/nd/gnrc_sixlowpan_nd.c

@@ -133,13 +133,14 @@ kernel_pid_t gnrc_sixlowpan_nd_next_hop_l2addr(uint8_t *l2addr, uint8_t *l2addr_
     next_hop = ipv6_ext_rh_next_hop(hdr);
 #endif
 #ifdef MODULE_FIB
+    kernel_pid_t fib_iface;
     ipv6_addr_t next_hop_actual;    /* FIB copies address into this variable */
     /* don't look-up link local addresses in FIB */
     if ((next_hop == NULL) && !ipv6_addr_is_link_local(dst)) {
         size_t next_hop_size = sizeof(ipv6_addr_t);
         uint32_t next_hop_flags = 0;
         if ((next_hop == NULL) &&
-            (fib_get_next_hop(&gnrc_ipv6_fib_table, &iface, next_hop_actual.u8, &next_hop_size,
+            (fib_get_next_hop(&gnrc_ipv6_fib_table, &fib_iface, next_hop_actual.u8, &next_hop_size,
                               &next_hop_flags, (uint8_t *)dst,
                               sizeof(ipv6_addr_t), 0) >= 0) &&
             (next_hop_size == sizeof(ipv6_addr_t))) {
@@ -150,6 +151,11 @@ kernel_pid_t gnrc_sixlowpan_nd_next_hop_l2addr(uint8_t *l2addr, uint8_t *l2addr_
 #ifdef MODULE_GNRC_SIXLOWPAN_ND_ROUTER
     /* next hop determination: https://tools.ietf.org/html/rfc6775#section-6.5.4 */
     nc_entry = gnrc_ipv6_nc_get(iface, dst);
+#ifdef MODULE_FIB
+    if ((next_hop != NULL) && (nc_entry == NULL)) {
+        nc_entry = gnrc_ipv6_nc_get(fib_iface, dst);
+    }
+#endif
     /* if NCE found */
     if (nc_entry != NULL) {
         gnrc_ipv6_netif_t *ipv6_if = gnrc_ipv6_netif_get(nc_entry->iface);