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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

A network engineer configures an 802.1Q trunk link between two switches, `SW-CORE-01` and `SW-ACCESS-01`. Interface `GigabitEthernet0/1` on `SW-CORE-01` is configured with a native VLAN of VLAN 10, while the corresponding interface on `SW-ACCESS-01` retains the default native VLAN of VLAN 1. Which of the following statements correctly describe the operational impact of this configuration? (Select TWO.)

  1. Untagged frames transmitted from VLAN 10 on `SW-CORE-01` will be placed into VLAN 1 upon arrival at `SW-ACCESS-01`.Answer
  2. Control plane protocols such as CDP or LLDP will trigger warning logs indicating a Native VLAN mismatch error.Answer
  3. C
    Frames explicitly tagged for VLAN 10 by `SW-CORE-01` will be automatically discarded by `SW-ACCESS-01` upon arrival.
  4. D
    Hosts on VLAN 10 and VLAN 1 will be able to perform Layer 3 routing between subnets without requiring a gateway device.

Answer

Untagged frames sent from VLAN 10 on `SW-CORE-01` will be assigned to VLAN 1 on `SW-ACCESS-01`, and management protocols like CDP/LLDP will log Native VLAN mismatch warnings.
When an 802.1Q native VLAN mismatch occurs, untagged traffic transmitted by one switch is placed into the native VLAN configured on the receiving switch, leaking traffic between VLAN 10 and VLAN 1. Additionally, discovery protocols like CDP or LLDP monitor link parameters and periodically report Native VLAN mismatch errors in syslog.

Step-by-Step Solution

1
Analyze how 802.1Q handles native VLAN frame tagging
Frames belonging to the native VLAN are sent untagged across an 802.1Q trunk link.
802.1Q standard leaves native VLAN frames unencapsulated for backward compatibility with non-tagging devices.
2
Determine frame processing on the receiving switch
The receiving switch (`SW-ACCESS-01`) inspects incoming untagged frames and assigns them to its own native VLAN (VLAN 1).
Switches map incoming untagged frames directly to whatever local native VLAN ID is configured on that specific trunk interface.
3
Identify administrative and protocol indicators of mismatch
CDP and LLDP detect mismatched configuration parameters and generate log alerts.
Neighbor discovery protocols continuously exchange interface configuration details to identify misconfigurations.

Key Concept

802.1Q Native VLAN Tagging and Mismatch Consequences
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Topics

VLAN Configuration and 802.1Q Trunking

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