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Difficulty: Very hardVLAN Configuration and 802.1Q Trunking

Match each 802.1Q trunking configuration scenario on the left with its corresponding frame encapsulation and ingress/egress processing behavior on the right.

  • An untagged ethernet frame arrives on an 802.1Q trunk interface configured with native VLAN 40.The switch implicitly maps the frame to VLAN 40 for internal forwarding; when egressing another trunk link where VLAN 40 is native, the frame is transmitted without an 802.1Q header.
  • An ethernet frame containing an 802.1Q tag with VLAN ID 0 (priority-tagged frame) arrives on a trunk port configured with native VLAN 40.The switch extracts the 802.1p Class of Service (CoS) bits for QoS prioritization and assigns the frame to the trunk port's configured native VLAN (VLAN 40) for forwarding.
  • An ethernet frame tagged with VLAN ID 40 arrives on an 802.1Q trunk interface where VLAN 40 is explicitly configured as the native VLAN.The frame is processed within VLAN 40, but because it arrived tagged on a port expecting untagged native frames, it may be dropped or generate a configuration warning depending on native tag settings.
  • An ethernet frame tagged with VLAN ID 10 arrives on a standard single-VLAN access port configured with access VLAN 10.The interface drops the frame upon ingress because standard access ports only accept untagged frames and reject incoming 802.1Q tagged frames.

Answer

Untagged frames on a trunk port map to the native VLAN and egress untagged on matching native trunks. Priority-tagged frames (VLAN 0) preserve 802.1p CoS bits while placing traffic into the native VLAN. Explicitly tagged native VLAN frames cause ingress processing anomalies unless native tagging is globally forced. Standard access ports drop incoming tagged frames.
Each match correctly reflects IEEE 802.1Q standards and switchport mechanics: untagged frames map directly to the native VLAN; VID 0 frames deliver 802.1p CoS priority to the native VLAN; explicitly tagged native frames violate default untagged expectations; and standard access ports reject tagged frames at ingress.

Step-by-Step Solution

1
Analyze untagged frame processing on trunk interfaces.
Untagged ingress traffic is assigned to the interface's configured native VLAN (VLAN 40) and transmitted untagged across native trunks.
IEEE 802.1Q backwards compatibility requires untagged traffic to be mapped to the native VLAN.
2
Analyze VLAN ID 0 (802.1p priority tagging) behavior.
VLAN ID 0 carries CoS priority bits but no VLAN ID, directing traffic to the native VLAN.
The 802.1Q header format reserves VID 0 exclusively for layer 2 QoS prioritization.
3
Analyze tagged native VLAN frame handling.
Receiving a tagged frame matching the native VLAN ID creates a mismatch condition unless tag native behavior is globally configured.
Standard 802.1Q trunking expects native VLAN traffic to be unencapsulated.
4
Evaluate ingress frame rules on access switchports.
Access ports drop 802.1Q tagged frames upon receipt.
Access interfaces operate purely in a single untagged broadcast domain.

Key Concept

802.1Q Tagging, Native VLAN Behavior, and Access vs Trunk Port Ingress Rules
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Topics

VLAN Configuration and 802.1Q Trunking

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