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Difficulty: HardVLAN Configuration and 802.1Q Trunking

A network administrator is designing and hardening VLAN trunking parameters across enterprise switches to ensure proper frame encapsulation, DTP negotiation, and VLAN prune/allow list configurations. Match each VLAN configuration parameter or frame header attribute on the left with its corresponding operational behavior on the right.

  • switchport trunk native vlan 99Specifies that egress frames belonging to this VLAN identifier are transmitted without an added 4-byte encapsulation header.
  • switchport trunk allowed vlan add 10,20Appends specific VLAN IDs to the interface egress permit list without replacing the currently allowed VLAN range.
  • switchport mode dynamic autoConfigures the interface to passively listen for DTP negotiation frames and transition to trunking only if requested by the remote neighbor.
  • IEEE 802.1Q Tag StructureInserts a 4-byte field containing a TPID of 0x8100, Priority Code Point bits, and a 12-bit VLAN Identifier between the Source MAC and EtherType fields.

Answer

Each configuration command or 802.1Q frame attribute corresponds to a specific operational mechanism: native VLAN designation handles untagged frame egress, the allowed list 'add' modifier preserves existing allowed VLAN definitions while appending new ones, dynamic auto modes passively negotiate trunking via DTP, and 802.1Q tagging places a 4-byte header containing VLAN ID and CoS fields immediately following the Ethernet Source MAC address.
The matching pairs correctly connect switch configuration commands and frame attributes to their exact operational outcomes: 'switchport trunk native vlan 99' specifies untagged frame handling for VLAN 99; 'switchport trunk allowed vlan add 10,20' preserves existing allowed VLANs while appending VLANs 10 and 20; 'switchport mode dynamic auto' passively waits for DTP requests; and the IEEE 802.1Q Tag Structure inserts a 4-byte header (TPID 0x8100 and TCI) between the Source MAC address and the EtherType field.

Step-by-Step Solution

1
Analyze native VLAN behavior under IEEE 802.1Q standard.
Identified that native VLAN traffic bypasses 802.1Q encapsulation and egresses the trunk port untagged.
802.1Q trunk ports send and expect native VLAN frames without a tag header for backward compatibility with unmanaged switches.
2
Evaluate CLI syntax for trunk allowed VLAN modification.
Determined that using 'switchport trunk allowed vlan add' appends entries without clearing existing allowed VLANs.
Omitting the 'add' parameter overwrites the entire permitted VLAN list, which is a common misconfiguration error.
3
Evaluate Dynamic Trunking Protocol (DTP) operational modes.
Matched dynamic auto to passive trunk negotiation.
Ports in dynamic auto mode respond to incoming DTP request frames from dynamic desirable or trunk ports but will not actively initiate negotiation.
4
Analyze Ethernet frame fields for IEEE 802.1Q encapsulation.
Matched 802.1Q header details to its 4-byte structure containing TPID 0x8100, PCP, DEI, and 12-bit VLAN ID.
The 802.1Q tag is inserted between the Source MAC address and the original EtherType/Length field in Ethernet frames.

Key Concept

VLAN Configuration, DTP Operational Modes, and 802.1Q Frame Header Tagging Mechanics
Estimated Time:2m 30s
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Topics

VLAN Configuration and 802.1Q Trunking

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