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Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

A network administrator configures an IEEE 802.1Q trunk link between two Cisco Catalyst switches. The trunk interface is configured to carry traffic for VLAN 10 (Data), VLAN 20 (configured as the native VLAN), and VLAN 30 (Voice). Which two statements accurately describe how traffic is processed over this trunk connection?

  1. Frames belonging to VLAN 10 and VLAN 30 have a 4-byte 802.1Q header inserted into the Ethernet frame prior to transmission across the trunk.Answer
  2. Frames belonging to VLAN 20 are transmitted untagged across the trunk link under default 802.1Q operational behavior.Answer
  3. C
    Untagged frames received on the trunk interface are automatically dropped by the switch to prevent security risks.
  4. D
    Configuring VLAN 20 as the native VLAN automatically removes VLAN 1 from the active allowed VLAN list on the trunk interface.

Answer

The two correct statements are that frames belonging to VLAN 10 and VLAN 30 have a 4-byte 802.1Q header inserted into the Ethernet frame, and frames belonging to VLAN 20 are transmitted untagged across the trunk link under default 802.1Q operational behavior.
IEEE 802.1Q trunking inserts a 4-byte header into frames belonging to tagged VLANs (such as VLAN 10 and VLAN 30) so recipient switches can identify their origin. Additionally, under standard 802.1Q operation, traffic belonging to the configured native VLAN (VLAN 20) is transmitted across the trunk link without a tag.

Step-by-Step Solution

1
Analyze standard IEEE 802.1Q encapsulation mechanics for tagged VLANs.
Frames belonging to non-native VLANs (VLAN 10 and VLAN 30) are tagged with a 4-byte 802.1Q header containing the Tag Protocol Identifier (TPID) and Tag Control Information (TCI).
802.1Q uses internal tagging to preserve VLAN identification across switch interconnects.
2
Evaluate the treatment of the designated native VLAN on an 802.1Q trunk link.
Frames associated with VLAN 20 (the native VLAN) are sent without an 802.1Q tag under default switch configurations.
IEEE 802.1Q natively supports untagged frames for backward compatibility with non-tagging legacy devices.
3
Evaluate ingress processing of untagged frames and native VLAN allowed list behavior.
Untagged ingress frames are mapped directly to the native VLAN rather than being dropped, and altering the native VLAN setting does not automatically prune VLAN 1.
Receptive untagged traffic maps directly to the port's native VLAN ID, while allowed VLAN lists require explicit configuration modifications.

Key Concept

802.1Q Trunking Frame Tagging and Native VLAN Operations
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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