diff --git a/core/contiki-default-conf.h b/core/contiki-default-conf.h
index 698b48286..0083f7e10 100644
--- a/core/contiki-default-conf.h
+++ b/core/contiki-default-conf.h
@@ -193,13 +193,21 @@
 #endif /* NBR_TABLE_CONF_MAX_NEIGHBORS */
 
 /* UIP_CONF_ND6_SEND_RA enables standard IPv6 Router Advertisement.
- * This is unneeded when RPL is used. */
+ * We enable it by default when IPv6 is used without RPL. */
 #ifndef UIP_CONF_ND6_SEND_RA
 #define UIP_CONF_ND6_SEND_RA (NETSTACK_CONF_WITH_IPV6 && !UIP_CONF_IPV6_RPL)
 #endif /* UIP_CONF_ND6_SEND_RA */
 
 /* UIP_CONF_ND6_SEND_NA enables standard IPv6 Neighbor Discovery Protocol.
-   This is unneeded when RPL is used. */
+   We enable it by default when IPv6 is used without RPL.
+   With RPL, the neighbor cache (link-local IPv6 <-> MAC address mapping)
+   is fed whenever receiving DIO and DAO messages. This is always sufficient
+   for RPL routing, i.e. to send to the preferred parent or any child.
+   Link-local unicast to other neighbors may, however, not be possible if
+   we never receive any DIO from them. This may happen if the link from the
+   neighbor to us is weak, if DIO transmissions are suppressed (Trickle
+   timer) or if the neighbor chooses not to transmit DIOs because it is
+   a leaf node or for any reason. */
 #ifndef UIP_CONF_ND6_SEND_NA
 #define UIP_CONF_ND6_SEND_NA (NETSTACK_CONF_WITH_IPV6 && !UIP_CONF_IPV6_RPL)
 #endif /* UIP_CONF_ND6_SEND_NA */