diff --git a/net/dsa/tag_8021q.c b/net/dsa/tag_8021q.c
index 6ebbd799c4eb61648f2466dc3d7498c7b1b954a4..67a1bc635a7b4342d0a7e935272776edf1295f7a 100644
--- a/net/dsa/tag_8021q.c
+++ b/net/dsa/tag_8021q.c
@@ -28,6 +28,7 @@
  *
  * RSV - VID[9]:
  *	To be used for further expansion of SWITCH_ID or for other purposes.
+ *	Must be transmitted as zero and ignored on receive.
  *
  * SWITCH_ID - VID[8:6]:
  *	Index of switch within DSA tree. Must be between 0 and
@@ -35,6 +36,7 @@
  *
  * RSV - VID[5:4]:
  *	To be used for further expansion of PORT or for other purposes.
+ *	Must be transmitted as zero and ignored on receive.
  *
  * PORT - VID[3:0]:
  *	Index of switch port. Must be between 0 and DSA_MAX_PORTS - 1.