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Difficulty: MediumTroubleshooting VLAN, Trunking, and Switching Issues

A network administrator is troubleshooting a connectivity failure between two switches connected via an 802.1Q trunk link. Host devices assigned to VLAN 10 on Switch-A are unable to communicate with servers on VLAN 10 connected to Switch-B. Inspection of the switch port configurations indicates that port GigabitEthernet 0/1 on Switch-A is configured with VLAN 10 as its native VLAN, while port GigabitEthernet 0/1 on Switch-B uses default VLAN 1 as its native VLAN. Which of the following best explains why VLAN 10 traffic fails to reach its destination across the trunk?

  1. Switch-A transmits VLAN 10 traffic untagged over the trunk, causing Switch-B to receive the frames and place them into VLAN 1 rather than VLAN 10.Answer
  2. B
    Switch-A automatically drops outgoing frames on VLAN 10 because devices on different VLANs cannot share the same physical trunk link without an inline Layer 3 router.
  3. C
    The native VLAN mismatch forces both trunk interface ports into half-duplex mode, causing excessive late collisions that drop all VLAN 10 frames.
  4. D
    Spanning Tree Protocol automatically disables the trunk interface on Switch-A because VLAN 10 has a higher numerical priority than VLAN 1.

Answer

Switch-A transmits VLAN 10 traffic untagged over the trunk, causing Switch-B to receive the frames and place them into VLAN 1 rather than VLAN 10.
The correct answer correctly identifies that an 802.1Q trunk sends frames belonging to its configured native VLAN without adding an 802.1Q VLAN tag. Because Switch-A has native VLAN 10 and Switch-B has native VLAN 1, Switch-A sends VLAN 10 frames untagged. Switch-B receives those untagged frames and places them into its own native VLAN (VLAN 1), preventing VLAN 10 hosts on Switch-A from reaching VLAN 10 servers on Switch-B.

Step-by-Step Solution

1
Analyze how 802.1Q trunks handle native VLAN traffic.
Frames associated with the native VLAN configured on a switch interface are sent across the trunk link without an IEEE 802.1Q VLAN tag.
802.1Q standard behavior dictates that native VLAN traffic remains untagged for backward compatibility.
2
Trace the path of untagged frames sent from Switch-A to Switch-B.
Switch-A sends VLAN 10 frames untagged because VLAN 10 is configured as native on Switch-A's trunk interface. Upon arrival at Switch-B, Switch-B receives untagged frames and assigns them to its native VLAN, which is configured as VLAN 1.
Switches assign all incoming untagged frames received on a trunk port to whichever VLAN is configured locally as the native VLAN.
3
Determine the operational impact on VLAN 10 connectivity.
VLAN 10 frames from Switch-A end up on VLAN 1 on Switch-B, preventing end-to-end Layer 2 communication between VLAN 10 hosts across the switches.
Traffic is misrouted into a different broadcast domain, breaking intra-VLAN communication.

Key Concept

802.1Q Native VLAN Tagging and Mismatch Behavior
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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