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Difficulty: HardVLAN Configuration and 802.1Q Trunking

A network administrator is auditing 802.1Q trunking and access port behavior on enterprise switches. Match each switchport operational scenario on the left with its corresponding 802.1Q frame processing or tagging outcome on the right.

  • An access port configured with 'switchport access vlan 20' receives an untagged Ethernet frame.The frame is internally mapped to VLAN 20 for processing within the local switch backplane.
  • A trunk port configured with 'switchport trunk native vlan 50' receives an untagged ingress frame.The untagged frame is implicitly associated with VLAN 50 upon entering the switchport.
  • A trunk port configured with 'switchport trunk native vlan 50' transmits an egress frame belonging to VLAN 50.The frame is transmitted across the physical link without inserting a 4-byte 802.1Q tag header.
  • A trunk port configured with 'switchport trunk allowed vlan 10,20' receives an ingress frame tagged with VLAN 30.The frame is dropped immediately at the ingress switchport due to VLAN membership filtering.

Answer

Each scenario matches as follows: Access port receiving untagged traffic maps to internal VLAN 20 processing; Trunk port receiving untagged traffic implicitly maps to native VLAN 50; Trunk port transmitting native VLAN 50 egresses untagged without an 802.1Q header; Trunk port receiving VLAN 30 on an allowed list of 10,20 drops the frame due to VLAN filtering.
The pairings correctly demonstrate fundamental 802.1Q trunking and access port behavior: access ports map untagged ingress frames to their access VLAN, trunk ports map untagged ingress frames to their native VLAN, native VLAN egress traffic is transmitted without an 802.1Q tag header, and frames tagged with VLANs outside the trunk allowed list are dropped at ingress.

Step-by-Step Solution

1
Analyze access port frame ingestion behavior.
Untagged ingress frames on an access port configured for VLAN 20 are logically mapped to VLAN 20.
Access ports only accept untagged frames (or frames with the matching access VLAN if voice VLAN is configured) and place them into the assigned access VLAN.
2
Analyze trunk port native VLAN ingress processing.
Untagged frames arriving on an 802.1Q trunk link are implicitly assigned to the native VLAN (VLAN 50).
IEEE 802.1Q design mandates that any untagged ingress frame arriving on a trunk interface belongs to the native VLAN configured on that interface.
3
Analyze trunk port native VLAN egress header tagging.
Frames assigned to VLAN 50 exit the trunk port untagged.
To maintain backward compatibility with non-802.1Q aware devices, 802.1Q trunks strip the 4-byte VLAN tag field for any frames belonging to the native VLAN prior to transmission.
4
Analyze trunk allowed VLAN list filtering rules.
A frame tagged with VLAN 30 arriving on a trunk restricted to VLANs 10 and 20 is dropped at ingress.
The 'switchport trunk allowed vlan' command defines the allowed list; any tagged frame carrying a VLAN ID not present in the allowed list is pruned/dropped by switch hardware.

Key Concept

802.1Q Frame Tagging, Native VLAN Ingress/Egress Rules, and Trunk Allowed List Pruning
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Topics

VLAN Configuration and 802.1Q Trunking

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