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Difficulty: Very hardVLAN Configuration and 802.1Q Trunking

A network administrator is troubleshooting an IEEE 802.1Q trunk link between two switches, SW-Access-A and SW-Access-B. On SW-Access-A, interface GigabitEthernet1/0/1 is configured as an 802.1Q trunk with native VLAN 20 and allowed VLANs 10, 20, and 30. On SW-Access-B, interface GigabitEthernet1/0/1 is configured as an 802.1Q trunk with native VLAN 1 and allowed VLANs 10, 20, and 30. Which of the following operational outcomes will occur on this trunk link? (Select TWO.)

  1. Untagged frames transmitted from VLAN 20 on SW-Access-A will be assigned to VLAN 1 upon reception on SW-Access-B.Answer
  2. Link-layer protocols such as CDP or LLDP will log native VLAN mismatch warnings on both switches.Answer
  3. C
    Tagged frames belonging to VLAN 10 will be dropped at the receiving interface due to the native VLAN configuration mismatch.
  4. D
    Traffic from VLAN 30 on SW-Access-A will automatically be routed into VLAN 10 on SW-Access-B through the trunk link.

Answer

The correct outcomes are: untagged frames transmitted from VLAN 20 on SW-Access-A will be assigned to VLAN 1 upon reception on SW-Access-B, and link-layer protocols such as CDP or LLDP will log native VLAN mismatch warnings on both switches.
When native VLAN IDs differ across an 802.1Q trunk link, untagged traffic sent by one switch's native VLAN is implicitly placed into the receiving switch's native VLAN, causing cross-VLAN traffic leakage. Additionally, neighbor discovery protocols like CDP or LLDP continuously exchange native VLAN parameters and log errors when a mismatch is detected.

Step-by-Step Solution

1
Analyze how 802.1Q trunking handles native VLAN traffic on transmission.
SW-Access-A strips 802.1Q tags from frames belonging to its native VLAN (VLAN 20) before forwarding them out GigabitEthernet1/0/1.
By definition in the IEEE 802.1Q standard, frames matching the local interface's native VLAN are sent untagged across trunk links.
2
Determine how the receiving switch processes untagged ingress frames.
SW-Access-B receives the untagged frames and places them into its locally configured native VLAN (VLAN 1).
An 802.1Q switch implicitly assigns all incoming untagged trunk frames to whatever VLAN ID is configured as native on that specific ingress interface.
3
Evaluate link-layer management protocol behaviors for misconfigured trunk parameters.
CDP or LLDP packets report mismatched native VLAN IDs (20 vs 1), generating console notification logs.
Discovery protocols periodically advertise interface metadata including native VLAN IDs, allowing network equipment to detect asymmetric trunk configurations.
4
Evaluate the impact on tagged VLAN traffic (VLAN 10 and VLAN 30).
Tagged traffic for allowed VLANs (10 and 30) continues to traverse the trunk properly and remains isolated.
Explicit 802.1Q tags are unaffected by native VLAN mismatches, and inter-VLAN routing remains required to move traffic between different VLAN numbers.

Key Concept

802.1Q Native VLAN Operation and Mismatch Behaviors
Estimated Time:2m 0s
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Topics

VLAN Configuration and 802.1Q Trunking

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