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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

A network administrator is configuring IEEE 802.1Q trunk links across enterprise switch interfaces. Match each trunking frame condition or interface operational scenario on the left with the corresponding switch processing or frame tagging behavior on the right.

  • An untagged Ethernet frame arrives at a trunk port configured with a native VLAN of 40.The frame is mapped to VLAN 40 and processed within that broadcast domain.
  • An egress Ethernet frame belonging to VLAN 10 leaves a trunk port where VLAN 10 is configured as the native VLAN.The frame is transmitted across the link without an 802.1Q header tag.
  • An egress Ethernet frame belonging to VLAN 20 leaves a trunk port where VLAN 10 is the native VLAN and VLAN 20 is in the allowed list.The frame is transmitted across the link with a 4-byte 802.1Q tag attached.
  • An ingress Ethernet frame carrying an 802.1Q tag for VLAN 99 arrives on a trunk port where VLAN 99 is not in the allowed VLAN list.The switch drops the frame at the ingress interface.

Answer

The untagged ingress frame maps to VLAN 40; the native VLAN egress frame is transmitted untagged; the non-native allowed VLAN egress frame is transmitted with an 802.1Q tag; and the non-allowed tagged ingress frame is dropped.
Each matching pair accurately reflects standard IEEE 802.1Q behavior: untagged ingress frames map to the native VLAN, egress native VLAN traffic stays untagged, egress traffic for other allowed VLANs receives a 4-byte 802.1Q header, and traffic for VLANs omitted from the allowed list is dropped at the port.

Step-by-Step Solution

1
Analyze native VLAN ingress processing for untagged frames.
Untagged frames arriving on an 802.1Q trunk interface are classified into the configured native VLAN (VLAN 40).
The 802.1Q standard dictates that untagged traffic is implicitly associated with the native VLAN ID assigned to the receiving port.
2
Determine egress tagging behavior for native VLAN traffic.
Frames belonging to the native VLAN (VLAN 10) are transmitted untagged across the trunk link.
By default, 802.1Q trunks strip or do not prepend a VLAN header tag for frames matching the link's native VLAN.
3
Determine egress tagging behavior for non-native allowed VLAN traffic.
Traffic for VLAN 20 is encapsulated with a standard 4-byte 802.1Q header containing the VLAN 20 tag.
All non-native VLAN traffic explicitly allowed on a trunk must be tagged so the downstream switch can maintain VLAN separation.
4
Evaluate filtering rules for non-allowed VLAN traffic.
The ingress frame for VLAN 99 is dropped because VLAN 99 is missing from the trunk's allowed VLAN list.
Trunk allowed VLAN lists restrict incoming and outgoing frame forwarding exclusively to permitted VLAN IDs.

Key Concept

802.1Q Trunk Tagging and Native VLAN Frame Handling
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Topics

VLAN Configuration and 802.1Q Trunking

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