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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

An administrator connects two switches, SW-ALPHA and SW-BETA, via an 802.1Q trunk link. Switch SW-ALPHA is configured with an 802.1Q trunking interface where VLAN 50 is explicitly defined as the native VLAN. Switch SW-BETA has a freshly unboxed configuration with default 802.1Q trunking parameters enabled (using VLAN 1 as the native VLAN). Trunking is operational on both sides. Which of the following technical outcomes will occur on this link? (Select TWO.)

  1. Untagged frames transmitted by SW-ALPHA from VLAN 50 will be placed into VLAN 1 upon arrival at SW-BETA.Answer
  2. B
    Frames tagged for VLAN 20 sent by SW-ALPHA will be dropped by SW-BETA due to the native VLAN configuration mismatch.
  3. Switches will generate CDP or syslog warning messages indicating a native VLAN mismatch condition.Answer
  4. D
    Traffic between devices on VLAN 50 and VLAN 1 will automatically be routed by the switches without requiring a Layer 3 device.

Answer

Untagged frames transmitted by the first switch from VLAN 50 will be placed into VLAN 1 upon arrival at the second switch, and the switches will generate CDP/syslog warnings indicating a native VLAN mismatch condition.
In 802.1Q trunking, native VLAN traffic is sent untagged across the link. When one switch transmits VLAN 50 traffic untagged and the receiving switch expects VLAN 1 traffic to be untagged, the receiving switch assigns those incoming untagged frames to VLAN 1. Additionally, management protocols such as CDP detect this parameter mismatch across neighboring ports and generate explicit syslog error alerts.

Step-by-Step Solution

1
Analyze how 802.1Q handles native VLAN traffic.
Frames belonging to the native VLAN are transmitted across the trunk link untagged (without an 802.1Q header).
802.1Q standard dictates that frames belonging to the port's native VLAN carry no 4-byte VLAN tag.
2
Evaluate ingress frame processing on the receiving switch.
When untagged frames arrive at the second switch, it assigns them to its own configured native VLAN (VLAN 1).
Switches implicitly map all incoming untagged trunk frames to their locally configured native VLAN ID.
3
Determine impact on tagged traffic and protocol logging.
Tagged traffic (VLAN 20) remains properly tagged and processed, while protocol daemons (like CDP) log native VLAN mismatch errors.
Native VLAN mismatches affect untagged frame mapping and trigger control-plane diagnostics, but do not stop processing of explicitly tagged frames.

Key Concept

802.1Q Native VLAN Operation and Mismatch Behavior
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Topics

VLAN Configuration and 802.1Q Trunking

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