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Difficulty: HardVLAN Configuration and 802.1Q Trunking

A network technician is troubleshooting an IEEE 802.1Q trunk link between two switches, `SW-DIST-01` and `SW-ACCESS-02`. The trunk is designed to transport traffic for VLAN 10 (Data), VLAN 20 (Voice), and VLAN 99 (Management), with VLAN 99 designated as the native VLAN.

On `SW-DIST-01`, interface GigabitEthernet1/0/10 is configured with the following commands:
text
interface GigabitEthernet1/0/10
switchport mode trunk
switchport trunk native vlan 99
switchport trunk allowed vlan 10,20,99

On `SW-ACCESS-02`, interface GigabitEthernet0/1 is configured as follows:
text
interface GigabitEthernet0/1
switchport mode trunk
switchport trunk allowed vlan 10,20

Management traffic for VLAN 99 fails to cross the trunk link, and switch log messages alert the administrator to a native VLAN mismatch. Which of the following best explains why VLAN 99 management traffic is failing across this link?

  1. SW-ACCESS-02 defaults to native VLAN 1 and drops egress VLAN 99 traffic because VLAN 99 is omitted from its allowed list, while incoming untagged native frames from SW-DIST-01 are incorrectly processed on VLAN 1.Answer
  2. B
    IEEE 802.1Q trunk interfaces automatically shut down unless all switches dynamically negotiate matching native VLAN IDs using Dynamic Trunking Protocol (DTP).
  3. C
    Untagged frames sent across an 802.1Q trunk are automatically dropped by the receiving switch unless 802.1Q tag header insertion is forcibly enabled for every configured VLAN.
  4. D
    Devices assigned to VLAN 99 on different switches cannot communicate across a trunk unless a Layer 3 router performs inter-VLAN routing between the two switch interfaces.

Answer

The connectivity failure occurs because SW-ACCESS-02 defaults to native VLAN 1 and omits VLAN 99 from its allowed VLAN list. Consequently, untagged native traffic sent from SW-DIST-01 (VLAN 99) is ingested on SW-ACCESS-02 into VLAN 1, while return frames belonging to VLAN 99 are pruned at the interface egress filter.
On an 802.1Q trunk, frames associated with the designated native VLAN are transmitted untagged. When SW-DIST-01 (configured with native VLAN 99) sends untagged frames, SW-ACCESS-02 (configured with default native VLAN 1) accepts them into VLAN 1. Furthermore, because SW-ACCESS-02 explicitly restricts allowed trunk traffic to VLANs 10 and 20 (`switchport trunk allowed vlan 10,20`), any frame tagged for VLAN 99 or destined for VLAN 99 across the trunk is pruned and dropped.

Step-by-Step Solution

1
Analyze the default native VLAN behavior on switch interfaces.
Unless explicitly configured with `switchport trunk native vlan <id>`, Cisco/IEEE switchports default to VLAN 1 as the native VLAN.
SW-DIST-01 transmits VLAN 99 traffic across the trunk untagged because VLAN 99 is specified as its native VLAN. SW-ACCESS-02 receives these untagged frames and maps them to its default native VLAN 1.
2
Analyze the allowed VLAN list configuration on SW-ACCESS-02.
The command `switchport trunk allowed vlan 10,20` explicitly restricts allowed traffic to VLANs 10 and 20, pruning VLAN 99.
Any frame originating from or destined to VLAN 99 on SW-ACCESS-02 is dropped at the trunk interface because VLAN 99 is excluded from the trunk's active allowed list.
3
Synthesize the dual failure mechanism resulting in native VLAN mismatch log errors.
Untagged ingress frames land on VLAN 1 instead of VLAN 99, and egress VLAN 99 frames are pruned, producing native VLAN mismatch log warnings and complete management unreachability.
Resolving this issue requires adding `switchport trunk native vlan 99` and appending VLAN 99 to the allowed list on SW-ACCESS-02.

Key Concept

802.1Q Trunking, Native VLAN Ingress/Egress Processing, and Allowed VLAN List Filtering
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Topics

VLAN Configuration and 802.1Q Trunking

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