diff --git a/ryu/lib/packet/pbb.py b/ryu/lib/packet/pbb.py
index 6e655e9d..7892436e 100644
--- a/ryu/lib/packet/pbb.py
+++ b/ryu/lib/packet/pbb.py
@@ -52,6 +52,7 @@ class itag(packet_base.PacketBase):
         dei = data >> 28 & 1
         uca = data >> 27 & 1
         sid = data & 0x00ffffff
+        # circular import: ethernet -> vlan -> pbb
         from ryu.lib.packet import ethernet
         return (cls(pcp, dei, uca, sid), ethernet.ethernet,
                 buf[cls._MIN_LEN:])