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Difficulty: HardInterswitch Connectivity and Trunking (802.1Q)

A network engineer is troubleshooting trunk connectivity between two switches and executes the following command on Switch-A:

Switch-A# show interfaces gigabitEthernet 1/0/2 trunk

Port Mode Encapsulation Status Native vlan
Gi1/0/2 auto n-802.1q trunking 99

Port VLANs allowed on trunk
Gi1/0/2 1-1005

Port VLANs allowed and active in management domain
Gi1/0/2 1,10,20,99

Port VLANs in spanning tree forwarding state and not pruned
Gi1/0/2 1,10,20

CDP emits periodic syslog messages warning of a native VLAN mismatch with the remote neighbor switch. Which statement accurately explains why VLAN 99 is absent from the spanning tree forwarding list and how untagged frames sent from Switch-A across this link are handled?

  1. Spanning Tree Protocol placed VLAN 99 into a PVID-inconsistency blocked state because the remote switch uses a different native VLAN, while Switch-A transmits VLAN 99 frames across the trunk without an 802.1Q tag.Answer
  2. B
    VLAN 99 is omitted from the forwarding list because it has not been created in the local switch VLAN database, causing untagged traffic to be dropped at the ingress interface.
  3. C
    Enabling PortFast on the trunk interface caused Spanning Tree Protocol to drop all tagged frame headers for VLAN 99 to prevent transient bridging loops.
  4. D
    Dynamic Trunking Protocol (DTP) negotiated the port into auto mode, which automatically prunes the native VLAN when CDP detects mismatched neighbor TLV parameters.

Answer

Spanning Tree Protocol placed VLAN 99 into a PVID-inconsistency blocked state because the remote switch uses a different native VLAN, while Switch-A transmits VLAN 99 frames across the trunk without an 802.1Q tag.
On an IEEE 802.1Q trunk, traffic belonging to the configured native VLAN is transmitted untagged. When connected switches have mismatched native VLAN configurations, Cisco Per-VLAN Spanning Tree (PVST+/Rapid PVST+) detects a Port VLAN ID (PVID) inconsistency through BPDUs. To prevent Layer 2 bridging loops and unauthorized traffic leaking between different VLANs, STP blocks the mismatched native VLAN on the trunk interface, omitting it from the 'VLANs in spanning tree forwarding state' list.

Step-by-Step Solution

1
Analyze the 'show interfaces trunk' output fields.
Interface Gi1/0/2 has negotiated 802.1Q trunking (mode auto, status trunking) with Native VLAN 99. VLAN 99 is listed under allowed VLANs and active management domain VLANs.
Confirms that VLAN 99 exists in the local VLAN database and is permitted on the trunk allowed list.
2
Compare active management VLANs against spanning tree forwarding VLANs.
VLAN 99 is present in the active management domain (1, 10, 20, 99) but missing from the spanning tree forwarding state list (1, 10, 20).
Identifies that Spanning Tree Protocol (STP) has placed VLAN 99 into a blocked state.
3
Evaluate the impact of the CDP native VLAN mismatch warning on 802.1Q trunk operations.
When native VLANs differ across an 802.1Q trunk link, PVST+/Rapid PVST+ detects a PVID inconsistency via BPDUs and puts the native VLAN into a PVID-inconsistency blocking state. Per IEEE 802.1Q standards, frames originating from the local native VLAN (VLAN 99) are transmitted without an 802.1Q tag.
Connects the symptoms of the syslog warning to STP blocking mechanics and 802.1Q tagging behavior.

Key Concept

802.1Q Native VLAN Untagged Framing and STP PVID Inconsistency Blocking
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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