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Difficulty: EasyTroubleshooting VLAN, Trunking, and Switching Issues

During a network audit, a technician discovers that Switch-1 has its trunk interface native VLAN set to VLAN 10, while Switch-2 on the opposite end of the trunk link has its native VLAN set to VLAN 20. Which of the following issues will occur as a direct result of this misconfiguration?

  1. Untagged frames transmitted by Switch-1 will be processed on VLAN 20 by Switch-2, causing cross-VLAN traffic leakage.Answer
  2. B
    Hosts connected to VLAN 10 and VLAN 20 will automatically obtain Layer 3 routing between subnets without a router.
  3. C
    The trunk interface will continuously log late collisions and frame check sequence (FCS) alignment errors.
  4. D
    Switch-1 will automatically become the STP root bridge because it has a lower native VLAN ID.

Answer

Untagged frames transmitted by Switch-1 will be processed on VLAN 20 by Switch-2, causing cross-VLAN traffic leakage.
Under 802.1Q encapsulation, frames in the native VLAN travel untagged across trunk links. When the receiving switch has a different native VLAN configured, it places all incoming untagged frames into its own configured native VLAN, causing traffic leakage between two distinct VLANs.

Step-by-Step Solution

1
Identify how 802.1Q trunking handles native VLAN traffic.
802.1Q trunks send frames belonging to the configured native VLAN without an 802.1Q header (untagged).
Native VLANs are designed to carry untagged control and legacy traffic across trunk links.
2
Analyze the impact of a native VLAN mismatch across the link.
Switch-1 sends untagged frames intended for VLAN 10. Switch-2 receives these untagged frames and assigns them to its own native VLAN (VLAN 20).
Switches assume any incoming untagged frame on a trunk interface belongs to their locally configured native VLAN.
3
Select the option that correctly describes this behavior.
Cross-VLAN traffic leakage occurs between VLAN 10 and VLAN 20.
This matches the technical consequence of an 802.1Q native VLAN mismatch.

Key Concept

Native VLAN Mismatch Behavior on 802.1Q Trunks
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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