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Difficulty: HardVLAN Configuration and 802.1Q Trunking

A network engineer is auditing switchport trunk configurations and frame tagging operations across enterprise switches. Match each IEEE 802.1Q trunking configuration scenario or operational event on the left with its corresponding frame behavior or interface outcome on the right.

  • Native VLAN mismatch across a link (Switch A configured with native VLAN 99, Switch B configured with native VLAN 1)Triggers CDP/PVST+ syslog error warnings and can result in improper cross-VLAN traffic leakage or STP port blocking
  • Ingress untagged Ethernet frame received on an 802.1Q trunk interface configured with native VLAN 20Assigned internally to VLAN 20 without prepending a 4-byte 802.1Q tag header
  • Execution of 'switchport trunk allowed vlan remove 50' on an active 802.1Q trunk interfacePrunes VLAN 50 frame traffic from traversing the trunk interface in both ingress and egress directions
  • Egress transmission of a frame assigned to VLAN 10 across an 802.1Q trunk link where native VLAN is set to 20Encapsulated with a 4-byte 802.1Q tag header containing Tag Protocol Identifier 0x8100 and VLAN ID 10

Answer

Each 802.1Q trunking scenario correctly pairs with its respective protocol error behavior, frame tagging rule, or allowed list pruning action as outlined in the solution steps.
Matching each event to its technical description demonstrates core 802.1Q trunking principles: Native VLAN mismatches lead to protocol inconsistencies and improper VLAN leakage; untagged ingress frames map directly to the configured native VLAN without header addition; removing a VLAN from the trunk allowed list prunes traffic for that VLAN in both directions; and non-native egress frames are encapsulated with 4-byte 802.1Q headers containing TPID 0x8100 and the corresponding VLAN ID.

Step-by-Step Solution

1
Analyze Native VLAN Mismatch Behavior
Configuring conflicting native VLAN IDs (VLAN 99 vs VLAN 1) causes untagged traffic to cross broadcast domains, triggering CDP and PVST+ mismatch alerts and potential STP port inconsistency states.
Both endpoints of an 802.1Q trunk must share an identical native VLAN ID to prevent cross-VLAN frame leakage.
2
Determine Ingress Untagged Frame Association
Untagged frames arriving on an 802.1Q trunk configured with native VLAN 20 are internally classified under VLAN 20.
IEEE 802.1Q specifies that unencapsulated ingress frames are assigned to the port's configured native VLAN.
3
Evaluate CLI Allowed VLAN List Removal Command
Running 'switchport trunk allowed vlan remove 50' prunes VLAN 50 from the active trunk pass-through list.
Allowed VLAN lists restrict which VLAN broadcast domain traffic is permitted to traverse a trunk interface.
4
Examine Egress Tagging for Non-Native VLANs
Frames belonging to VLAN 10 sent over a trunk with native VLAN 20 must be tagged with a 4-byte 802.1Q header containing TPID 0x8100 and VLAN ID 10.
All traffic not belonging to the native VLAN requires explicit 802.1Q tagging to maintain VLAN identity across network switches.

Key Concept

802.1Q Trunk Tagging, Native VLAN Processing, and Allowed List Pruning
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Topics

VLAN Configuration and 802.1Q Trunking

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