How to Resolve Inter-Switch VLAN Communication Failure?

350-401 By islever Editorial Last updated

Short answer

A Set the switchport mode to dynamic desirable on Switch B.

Question

Refer to the exhibit. Users in the Operations VLAN on Switch A are unable to communicate with users in the Operations VLAN on Switch B. Which action resolves the issue? - Exhibit: How to Resolve Inter-Switch VLAN Communication Failure?
  1. Set the switchport mode to dynamic desirable on Switch B. Correct Answer
  2. Set the EtherChannel mode to LACP on Switch A.
  3. Configure the same allowed VLAN list on Switch A and Switch B.
  4. Set the EtherChannel mode to PAGP on Switch B.

Key Takeaways

The question tests whether candidates recognize that an auto/auto DTP pairing prevents trunk establishment, creating a trap where candidates incorrectly prioritize VLAN filtering over basic Layer 2 negotiation.

This question evaluates Dynamic Trunking Protocol (DTP) negotiation behavior between connected switches. The community consensus confirms that changing one interface from dynamic auto to dynamic desirable forces trunk formation, enabling tagged VLAN traffic to pass.

Common mistake: Many candidates select Option C, attempting to configure identical allowed VLAN lists. They overlook that VLAN permits are irrelevant until a trunk link is successfully formed through DTP negotiation.

Expert Analysis

Understanding the Core Issue

The scenario describes a failure of VLAN 30 (Operations) traffic to traverse between Switch A and Switch B. Based on the options and community feedback, the connecting interfaces are configured with switchport mode dynamic auto on both sides. According to Cisco’s DTP specification, when both ends are set to auto, neither switch initiates trunk negotiation. Consequently, the link defaults to access mode, carrying only untagged traffic on the native VLAN. Since VLAN 30 frames are 802.1Q tagged, they are dropped at the Layer 2 boundary.

Why Option A is Correct

Changing Switch B’s interface to dynamic desirable forces DTP negotiation. The desirable state actively sends DTP frames requesting trunk formation. When paired with auto on Switch A, the negotiation succeeds, establishing a trunk link. Once the trunk is up, tagged VLAN 30 traffic flows freely, resolving the isolation. This directly follows the Cisco DTP matrix: auto/desirable equals Trunk.

Why Other Options Fail

  • Option B & D (EtherChannel Modes): LACP and PAgP bundle multiple physical links into a single logical channel. The question does not indicate a multi-link aggregation requirement, and altering EtherChannel protocols cannot override a failed DTP negotiation on a single link.
  • Option C (Allowed VLAN List): While matching allowed VLANs is standard practice, it is functionally useless if the link isn’t trunking. As noted by multiple community experts, trunk formation must precede VLAN filtering; configuring permit lists on an access port has zero impact.

Community Insights & Clarifications

The vote distribution heavily favors Option A (80%), with seasoned engineers emphasizing the DTP hierarchy. One candidate correctly highlighted that auto/auto yields an access link, while another clarified that Option C addresses EtherChannel consistency rather than basic trunk negotiation. A minor debate exists regarding whether Switch B was already desirable in the exhibit, but the prevailing interpretation confirms the image shows auto on both sides, making A the definitive fix.

Exam Strategy

Always verify Layer 2 trunk negotiation status before diving into VLAN-specific configurations. Remember the DTP rule of thumb: auto waits, desirable asks, and trunk forces. When troubleshooting inter-switch connectivity, confirm the link state and DTP mode first to avoid wasting time on secondary settings like allowed VLAN lists or EtherChannel parameters.

Community Votes

A
80%
C
20%

80% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 8 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 8

  1. Swiz005 👍 3 Answered A
    Operations VLAN is 30. and VLAN 30 exists on both switches. Makes A the most sensible option
  2. yasmiine 👍 2
    A is OK, but C is OK too, the etherchannel must have the same allowed vlan (1,10,20,30,50) in switch B. Am i wrong ?
  3. shefo1 👍 1 Answered C
    why A is wrong :: A. Set the switchport mode to dynamic desirable on Switch B: This option would not resolve the issue. The switchport mode is already set to dynamic auto on both switches, which is the correct setting for a trunk link.
  4. Miro07 👍 1 Answered A
    A seems to be correct We need a trunk to form before VLANs can communicate. Option C is condition for PAGP to create etherchannel but is not necessary for VLANs to communicate.
  5. Zeruz 👍 1 Answered A
    auto and auto won't make a trunking. At least one of the switches has to be configured in desireable to form a trunk. Answer A is the correct option.
Show 3 more comments
  1. SAAVYTECH 👍 1 Answered A
    People need to differentiate between channel group mode and Switchport trunking modes.
  2. Mohaned990_go 👍 1 Answered C
    c is correct A incorrect because switch B it is desirable already
  3. chiacche 👍 1 Answered A
    auto -> desirable

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