Which of the following VLANs can you configure on an FXO port?

A.
no VLANs

B.
data VLANs

C.
native VLANs

D.
voice VLANs

Explanation:
There are no virtual LANs (VLANs) that you can configure on a foreign exchange office (FXO) port. FXO ports
connect a voice gateway to the public switched telephone network (PSTN) or to a private branch exchange
(PBX). A voice gateway is a router that is responsible for call processing and routing between the PSTN and a
Voice over IP (VoIP) network, including the translation of analog voice signals to digital packets and vice versa.
You can configure voice VLANs only on Layer 2 access ports on a Cisco switch. Creating voice VLANs on a
switch enables the separation of voice traffic from data traffic on a network. To enable the voice VLAN feature
on a Cisco switch, you should issue the switchport voice vlan {vlan-id | dot1p | none | untagged} command in
interface configuration mode. The dot1p keyword configures voice traffic to be sent with a default 802.1p priority
of 5 and to use VLAN 0 as the VLAN ID.
The switchport voice vlan vlan-id command configures voice traffic to be tagged and sent over a user –
specified voice VLAN. Voice traffic will be carried in 802.1Q frames and will be carried on a different VLAN than
data traffic. For example, if you issue the switchport voice vlan 2 command, voice traffic will be tagged with
802.1Q information and sent over VLAN 2.
Similar to the switchport voice vlan untagged command, the switchport voice vlan none command configures
voice traffic to be untagged and sent over the same VLAN as data traffic, which is the native VLAN. When the
none keyword is used, voice traffic does not use 802.1p priority tagging or Class of Service (CoS), and voicetraffic is transmitted with data traffic.
The switchport voice vlan untagged command configures voice traffic to be untagged and sent over the native
VLAN. However, if data and voice devices are configured to operate on the same VLAN, the voice traffic can
experience quality problems, such as jitter or choppiness. Untagged traffic is sent without 802.1Q
encapsulation. When the switchport voice vlan untagged command is issued, both voice traffic and data traffic
are transmitted over the native VLAN. You do not need to specify a voice VLAN when the switchport voice vlan
untagged command is used.
You can configure data VLANs and native VLANs on both trunk ports and access ports on a Cisco switch. To
configure a data VLAN or the native VLAN on an access port, you should issue the switchport access vlan vlanid command, where vlan-id is the ID of the data VLAN or the native VLAN you want to configure. To configure a
trunk port with the native VLAN, you should issue the switchport trunk native vlan vlan-id command, where vlanid is the ID of the native VLAN. In addition, trunk ports are by default configured to allow traffic from all data
VLANs that are configured on the switch. You can issue the switchport trunk allowed vlan remove vlan-id list
command to specifically remove a list of data VLANs from a trunk port. You can add a specific VLAN to a trunk
port by issuing the switchport trunk allowed vlan add vlan-id list command, where vlan-id list is a list of the
VLAN IDs you want to add.

Cisco: Understanding Foreign Exchange Office (FXO) Voice Interface Cards
Cisco: Configuring Voice VLAN: Configuring Voice VLAN