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Difficulty: MediumTroubleshooting VLAN, Trunking, and Switching Issues

A network technician configures an 802.1Q trunk link between Switch-1 and Switch-2. Switch-1 is configured with native VLAN 10, whereas Switch-2 is configured with native VLAN 20. Users connected to VLAN 10 on Switch-1 report that they can unexpectedly reach servers on VLAN 20 attached to Switch-2 without going through a router, while console logs generate periodic CDP error messages. Which of the following best explains the root cause of this cross-VLAN communication?

  1. Untagged frames sent from VLAN 10 on Switch-1 travel across the trunk link and are automatically placed into native VLAN 20 upon arrival at Switch-2.Answer
  2. B
    A duplex mismatch on the trunk port is corrupting frame headers and causing the switches to route traffic between distinct VLAN broadcast domains.
  3. C
    The switches lack a Layer 3 routing engine, which forces all connected switch ports to bridge broadcast domains together by default.
  4. D
    Spanning Tree Protocol (STP) has selected Switch-2 as the root bridge, automatically merging VLAN 10 and VLAN 20 into a single spanning tree instance.

Answer

Untagged frames sent from VLAN 10 on Switch-1 travel across the trunk link and are automatically placed into native VLAN 20 upon arrival at Switch-2.
Under 802.1Q encapsulation, frames originating from the native VLAN are sent across a trunk link untagged. When neighboring switches have mismatched native VLAN settings (VLAN 10 on one end and VLAN 20 on the other), untagged frames originating on VLAN 10 are ingested by the receiving switch and mapped directly into VLAN 20. This creates a security leak and unexpected Layer 2 connectivity between two distinct VLANs.

Step-by-Step Solution

1
Analyze the 802.1Q trunking behavior regarding native VLANs.
802.1Q trunking transmits frames belonging to the native VLAN without inserting an 802.1Q VLAN tag.
Native VLAN traffic is intentionally untagged to maintain backward compatibility with legacy non-tagging devices.
2
Trace the path of untagged frames from Switch-1 to Switch-2.
Frames from VLAN 10 leave Switch-1 untagged. Switch-2 receives the untagged frames and assigns them to its local native VLAN (VLAN 20).
Switches implicitly assume all incoming untagged frames on an 802.1Q trunk belong to whichever VLAN is configured as native on that specific ingress interface.
3
Identify the diagnostic indicators matching the scenario.
Cross-VLAN traffic leakage without a router combined with CDP native VLAN mismatch log warnings confirms a native VLAN configuration mismatch.
Cisco Discovery Protocol (CDP) and LLDP continuously monitor link parameters and report native VLAN mismatches to prevent security and routing anomalies.

Key Concept

802.1Q Native VLAN Mismatch Dynamics
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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