The Port Fast feature is automatically enabled when voice VLAN is configured. When you disable voice VLAN, the Port Fast feature is not automatically disabled.
802.1Q VLAN frames are distinguished from ordinary Ethernet frames by the insertion of a 4-byte VLAN tag into the Ethernet header.
Because the 802.1Q tunneling feature increases the frame size by 4 bytes when the metro tag is added, you must configure all switches in the service-provider network to be able to process maximum frames by increasing the switch system MTU size to at least 1504 bytes.
The VLAN ID field inside an 802.1q frame consists of 12 bits. Therefore we have 212 = 4096 VLAN IDs, theoretically.
Each access port can be only assigned to one VLAN via the “switchport access vlan ” command.
This command is used to enable tagging of native VLAN frames on all 802.1Q trunk ports.
Answer A is not correct because even when the native VLAN is set to 1, all of the frames of the native VLAN are tagged.
Answer B is not correct because the control traffic still passes via the default VLAN (VLAN 1).
Answer C is not correct because all the frames are tagged with 4-byte dot1q tag.
Only answer D is best choice because control traffic (like CDP, VTP, STP, DTP…) uses VLAN 1 for communication. When the native VLAN is tagged (VLAN 1 by default) all control traffic is tagged too. If the native VLAN is not VLAN 1 then all the control traffic on VLAN 1 is still tagged by default (without using above command).
When you delete a VLAN, any LAN ports configured as access ports assigned to that VLAN become inactive. The ports remain associated with the VLAN (and inactive) until you assign them to a new VLAN.
The PortFast feature is automatically enabled when voice VLAN is configured. When you disable voice VLAN, the PortFast feature is not automatically disabled.