diff --git a/include/uapi/linux/neighbour.h b/include/uapi/linux/neighbour.h
index dc8b7220..00a60695 100644
--- a/include/uapi/linux/neighbour.h
+++ b/include/uapi/linux/neighbour.h
@@ -66,8 +66,11 @@ enum {
 #define NUD_NONE	0x00
 
 /* NUD_NOARP & NUD_PERMANENT are pseudostates, they never change
-   and make no address resolution or NUD.
-   NUD_PERMANENT also cannot be deleted by garbage collectors.
+ * and make no address resolution or NUD.
+ * NUD_PERMANENT also cannot be deleted by garbage collectors.
+ * When NTF_EXT_LEARNED is set for a bridge fdb entry the different cache entry
+ * states don't make sense and thus are ignored. Such entries don't age and
+ * can roam.
  */
 
 struct nda_cacheinfo {