What is the function of the command switchport trunk native vlan 999 on a Cisco Catalyst switch?

Question

Options

• AIt creates a VLAN 999 interface.
• BIt designates VLAN 999 for untagged traffic.
• CIt blocks VLAN 999 traffic from passing on the trunk.
• DIt designates VLAN 999 as the default for all unknown tagged traffic.

• A
  8% (2)
• B
  88% (21)
• C
  4% (1)

Explanation

switchport trunk native vlan 999 configures VLAN 999 as the native VLAN on a trunk link, meaning any traffic arriving untagged on that trunk is assigned to VLAN 999 - making B correct. Option A is wrong because this command doesn't create a Layer 3 VLAN interface (that requires interface vlan 999). Option C is the opposite of what happens - the native VLAN isn't blocked; it's the one that gets passed through without a tag. Option D confuses native VLAN with a catch-all for unknown tagged traffic; tagged frames that don't match a configured VLAN are simply dropped, not redirected to the native VLAN.

Memory tip: Think of the native VLAN as the trunk's "bare" lane - traffic riding in it travels without a tag (naked/untagged), like a car in a toll lane that doesn't need a ticket.

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