Index: documentation/content/en/books/handbook/advanced-networking/_index.adoc
===================================================================
--- documentation/content/en/books/handbook/advanced-networking/_index.adoc
+++ documentation/content/en/books/handbook/advanced-networking/_index.adoc
@@ -2417,6 +2417,16 @@
 
 Additional VLANs may be added, by simply adding the tag to the `vlans_em0` field and adding an additional line configuring the network on that VLAN tag's interface.
 
+[NOTE]
+====
+When defining VLANs in [.filename]#/etc/rc.conf#, make sure that the parent network interface is configured and enabled as well. The minimum configuration for the above example would be:
+
+[.programlisting]
+....
+ifconfig_em0="up"
+....
+====
+
 It is useful to assign a symbolic name to an interface so that when the associated hardware is changed, only a few configuration variables need to be updated.
 For example, security cameras need to be run over VLAN 1 on `em0`.
 Later, if the `em0` card is replaced with a card that uses the man:ixgb[4] driver, all references to `em0.1` will not have to change to `ixgb0.1`.