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Difficulty: EasyVLAN Configuration and 802.1Q Trunking

A network administrator is configuring an IEEE 802.1Q trunk link between two switches in an enterprise building. Which of the following statements correctly describe the standard operational behavior of 802.1Q trunking and native VLANs on this interconnect? (Select TWO.)

  1. 802.1Q trunking inserts a 4-byte tag into the Ethernet frame header to identify traffic belonging to non-native VLANs.Answer
  2. Untagged Ethernet frames received on an 802.1Q trunk port are implicitly assigned to the native VLAN configured on that port.Answer
  3. C
    Hosts residing on different VLANs across the trunk link can communicate directly at Layer 2 without requiring a routing device.
  4. D
    A native VLAN mismatch between the connected trunk interfaces causes the switch to immediately drop all tagged frames for all VLANs.

Answer

802.1Q trunking inserts a 4-byte tag into non-native VLAN Ethernet headers, and untagged frames arriving on a trunk port are implicitly assigned to the configured native VLAN.
IEEE 802.1Q defines trunking behavior by tagging frames with a 4-byte header for non-native VLANs while sending native VLAN frames without tags, allowing the receiving switch port to map untagged ingress frames to its local native VLAN.

Step-by-Step Solution

1
Analyze frame tagging mechanisms under IEEE 802.1Q.
IEEE 802.1Q adds a 4-byte field containing Tag Protocol Identifier (TPID) and Tag Control Information (TCI) to preserve VLAN identity across trunk links for non-native VLANs.
This tag allows switches on both ends of the trunk link to multiplex multiple VLANs over a single physical link.
2
Analyze native VLAN default processing behavior.
Native VLAN frames travel across the trunk link untagged. The receiving trunk port automatically assigns untagged frames to its configured native VLAN.
This behavior maintains backward compatibility with non-VLAN-aware legacy equipment.
3
Evaluate distractor claims against Layer 2 isolation and native VLAN mismatch symptoms.
VLANs strictly isolate Layer 2 broadcast domains, requiring a Layer 3 router for inter-VLAN forwarding. Furthermore, native VLAN mismatches cause traffic leakage into the mismatched VLAN rather than dropping tagged traffic.
Correctly identifying Layer 2 boundaries and trunk mismatch behaviors confirms the valid options.

Key Concept

802.1Q Frame Tagging and Native VLAN Processing
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Topics

VLAN Configuration and 802.1Q Trunking

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