Which of the following cannot be configured on a trunk port on a Cisco switch?
A.
a data VLAN
B.
a native VLAN
C.
a voice VLAN
D.
neither a voice VLAN nor a data VLAN
Explanation:
You cannot configure a voice virtual LAN (VLAN) on a trunk port on a Cisco switch. Although you can issue the
switchport voice vlan command on a trunk port, the voice VLAN will not be configured on the port unless you
also issue the switchport mode access command. Voice VLANs can be configured only on Layer 2 access
ports. Creating voice VLANs on a switch enables the separation of voice traffic from data traffic on a network. 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.
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 untagged command configures voice traffic to be untagged and sent over the native
VLAN. 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.
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 thandata 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 voice
traffic is transmitted with data traffic.
You can configure a data VLAN and the native VLAN on a trunk port. To configure a trunk port with the native
VLAN, you should issue the switchport trunk native vlan vlan-id command, where vlan-id 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: Configuring Voice VLAN: Configuring Voice VLAN