--- openflow-2011-09-19-3.h	2015-07-23 09:54:16.640479341 -0700
+++ openflow-2011-09-19-4.h	2015-07-23 09:54:16.644479341 -0700
@@ -777,7 +777,7 @@
  * outermost tag.  For a packet with no 802.1Q header, this has value
  * 0.
  *
- * Prereqs: None.
+ * Prereqs: OXM_OF_VLAN_VID must be different from OFPVID_NONE.
  *
  * Format: 8-bit integer in network byte order.
  *