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Difficulty: MediumSwitching Concepts and VLANs

A network administrator connects two Layer 2 switches via an 802.1Q trunk port. Switch A is configured with native VLAN 1010, while Switch B is configured with native VLAN 2020. If a host assigned to VLAN 1010 sends an untagged frame into Switch A that is forwarded over the trunk, how will Switch B handle the frame upon arrival?

  1. Switch B will accept the untagged frame and forward it to hosts in VLAN 20.Answer
  2. B
    Switch B will automatically drop the frame because devices in different VLANs require a Layer 3 router to exchange data.
  3. C
    Switch B will encapsulate the Layer 2 frame into a Layer 3 IP packet to route it to VLAN 10.
  4. D
    Switch B will change its Spanning Tree Protocol root bridge priority to match VLAN 10.

Answer

Switch B will accept the untagged frame and process it within VLAN 20, leading to a native VLAN mismatch leak.
Under 802.1Q trunking, traffic associated with the native VLAN is sent across the link untagged. When Switch A transmits a VLAN 10 frame untagged, Switch B accepts the untagged frame and assigns it to its local native VLAN (VLAN 20), resulting in unintended traffic leakage between VLAN 10 and VLAN 20.

Step-by-Step Solution

1
Analyze trunk encapsulation behavior for the native VLAN on Switch A
Switch A receives the frame on VLAN 10. Since VLAN 10 is configured as the native VLAN on Switch A's trunk port, Switch A transmits the frame across the trunk without adding an 802.1Q VLAN header tag.
By definition, IEEE 802.1Q does not tag frames associated with the native VLAN.
2
Analyze frame reception behavior on Switch B
Switch B receives an untagged frame on its trunk port. Switch B assigns all incoming untagged frames to its own configured native VLAN, which is VLAN 20.
Switches map incoming untagged trunk traffic directly to their locally configured native VLAN.
3
Determine the resulting network behavior
Traffic originating on VLAN 10 is delivered into VLAN 20, creating a security and operational issue known as native VLAN mismatch frame leakage.
Mismatched native VLAN configurations break Layer 2 broadcast domain isolation across trunk links.

Key Concept

802.1Q Native VLAN Trunking and Mismatch Behavior
Estimated Time:1m 15s
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Topics

Switching Concepts and VLANs

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