Handbook/Advanced Networking: Add note about VLAN creation

When adding a new VLAN, the parent interface must be up.  Add notes to
remind readers of this.

PR:             270942
Reviewed by:    jrm, leres
Differential Revision:  https://reviews.freebsd.org/D43993

1 files changed, 22 insertions, 0 deletions

diff --git a/documentation/content/en/books/handbook/advanced-networking/_index.adoc b/documentation/content/en/books/handbook/advanced-networking/_index.adoc
index 1a6e239cf7..a1f861079b 100644
--- a/documentation/content/en/books/handbook/advanced-networking/_index.adoc
+++ b/documentation/content/en/books/handbook/advanced-networking/_index.adoc
@@ -2406,6 +2406,17 @@ To configure VLANs at run time, with a NIC of `em0` and a VLAN tag of `5` the co
 See how the interface name includes the NIC driver name and the VLAN tag, separated by a period? This is a best practice to make maintaining the VLAN configuration easy when many VLANs are present on a machine.
 ====
 
+[NOTE]
+====
+When defining VLANs, ensure that the parent network interface is also configured and enabled.
+The minimum configuration for the above example would be:
+
+[source,shell]
+....
+# ifconfig em0 up
+....
+====
+
 To configure VLANs at boot time, [.filename]#/etc/rc.conf# must be updated.
 To duplicate the configuration above, the following will need to be added:
 
@@ -2417,6 +2428,17 @@ ifconfig_em0_5="inet 192.168.20.20/24"
 
 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`.