diff --git a/netbsd/sys/sys/mbuf.h b/netbsd/sys/sys/mbuf.h
index 1ee9557afa..9efa9cfe0d 100644
--- a/netbsd/sys/sys/mbuf.h
+++ b/netbsd/sys/sys/mbuf.h
@@ -89,6 +89,11 @@
  * at least MINCLSIZE of data must be stored.
  */
 
+/*
+ * NOTE: MINCLSIZE is changed to MHLEN + 1, to avoid allocating chained
+ * non-external mbufs in the driver.  This has no impact on performance
+ * seen from the packet statistics, and avoid header pullups in network code.
+ */
 #define	MLEN		(MSIZE - sizeof(struct m_hdr))	/* normal data len */
 #define	MHLEN		(MLEN - sizeof(struct pkthdr))	/* data len w/pkthdr */