Explanation of 'switchport trunk native vlan 999' Command

A

The correct answer is A. It designates VLAN 999 for untagged traffic.

The switchport trunk native vlan command is used to configure the native VLAN on a trunk port. The native VLAN is the VLAN that is used for untagged traffic on a trunk link.

When a packet enters a trunk port without a VLAN tag, it is assumed to belong to the native VLAN. By default, the native VLAN is VLAN 1. However, it is a security risk to use VLAN 1 as the native VLAN, as it can allow unauthorized access to the network.

The switchport trunk native vlan command allows you to change the native VLAN to a different VLAN. In this case, the command switchport trunk native vlan 999 is configuring VLAN 999 as the native VLAN on the trunk port. Any untagged traffic that enters the port will be assigned to VLAN 999.

Option B, "It blocks VLAN 999 traffic from passing on the trunk," is incorrect. The switchport trunk native vlan command does not block any traffic.

Option C, "It creates a VLAN 999 interface," is also incorrect. The switchport trunk native vlan command does not create an interface. It simply assigns the specified VLAN as the native VLAN on the trunk port.

Option D, "It designates VLAN 999 as the default for all unknown tagged traffic," is also incorrect. The switchport trunk native vlan command only affects untagged traffic. It does not affect tagged traffic.