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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

Match each IEEE 802.1Q trunking component or configuration behavior to its correct technical description.

  • Native VLAN frame egress behaviorFrames associated with this specific VLAN ID are transmitted across the trunk link without an inserted 4-byte header.
  • 802.1Q frame tagging mechanismInserts a 4-byte field containing a TPID of 0x8100 and a 12-bit VLAN Identifier between the Source MAC and EtherType fields.
  • Allowed VLAN list filteringSpecifies which VLAN IDs are permitted to send and receive traffic across the trunk interface, blocking unlisted VLANs.
  • Native VLAN mismatch conditionOccurs when interconnecting switch ports have conflicting native VLAN IDs, leading to potential VLAN hopping or STP inconsistencies.

Answer

Native VLAN frame egress behavior matches with un-tagged frame transmission. 802.1Q frame tagging mechanism matches with inserting a 4-byte field containing a 12-bit VLAN ID. Allowed VLAN list filtering matches with restricting allowed VLAN IDs across the trunk interface. Native VLAN mismatch condition matches with traffic leakage across differing native VLAN IDs and protocol warnings.
Each trunking feature maps to its core operational behavior under 802.1Q: native VLAN frames travel untagged; standard tagged frames carry a 4-byte 802.1Q header; allowed lists define authorized VLAN IDs; and native VLAN mismatches lead to cross-VLAN traffic leaking and administrative errors.

Step-by-Step Solution

1
Analyze IEEE 802.1Q standard frame format requirements.
Identified that standard 802.1Q trunking inserts a 4-byte tag into frames for tagged VLANs, whereas native VLAN frames are transmitted untagged.
Understanding frame header modifications is essential for troubleshooting trunk link behavior.
2
Evaluate trunk interface filtering mechanisms.
Determined that allowed VLAN lists act as interface-level filters to prune unnecessary VLAN traffic from traversing trunk connections.
Restricting allowed VLANs conserves bandwidth and enhances network security across switch trunks.
3
Examine the operational impact of configuration discrepancies.
Confirmed that mismatched native VLAN configurations cause traffic sent untagged from one VLAN to be processed by the remote switch on a different VLAN.
Native VLAN consistency is critical to prevent unwanted broadcast domain merging and STP topology instability.

Key Concept

802.1Q Trunking Fundamentals and Native VLAN Processing
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Topics

VLAN Configuration and 802.1Q Trunking

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