CCNA Switching and Network Access Practice Question
A network engineer notices that a newly connected switch-to-switch link is up, but traffic from multiple VLANs is not passing. When issuing the show interfaces trunk command, no trunk ports are listed. Both switch ports are configured with switchport mode dynamic auto. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the subtle difference between dynamic auto (passive) and dynamic desirable (active) DTP modes, leading candidates to incorrectly assume that two dynamic auto ports will automatically form a trunk.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Both ports are set to dynamic auto, so neither switch initiates DTP negotiation.
When both switch ports are configured with switchport mode dynamic auto, neither actively initiates Dynamic Trunking Protocol (DTP) negotiation. DTP dynamic auto ports wait for the other side to send DTP frames to form a trunk; since both sides are passive, the link remains in access mode and does not become a trunk, preventing traffic from multiple VLANs from passing.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
There is a native VLAN mismatch between the two switches.
Why it's wrong here
A native VLAN mismatch does not prevent a trunk from forming. DTP negotiates trunking irrespective of native VLAN assignment. A mismatch will allow the trunk to come up but cause CDP/STP notifications and possible untagged traffic leakage, yet the trunk interface appears in show interfaces trunk.
- ✗
The connecting cable is a straight-through Ethernet cable rather than a crossover cable.
Why it's wrong here
Modern Cisco switches support auto-MDIX, which automatically adjusts the interface to work with either a straight-through or crossover cable. A cable type mismatch would not prevent the link from coming up or forming a trunk. The link is already up, so this is not the cause.
- ✗
One switch lacks a VLAN that exists on the other switch.
Why it's wrong here
Trunk formation does not require identical VLAN databases. Trunk negotiation uses DTP, not VLAN presence. A missing VLAN would result in traffic for that VLAN being discarded or not forwarded, but the trunk itself would still be established and shown as a trunk port.
- ✓
Both ports are set to dynamic auto, so neither switch initiates DTP negotiation.
Why this is correct
In dynamic auto mode, a switch port passively waits for DTP negotiation requests. When both ends are dynamic auto, no side initiates the negotiation, so the ports default to access mode. This perfectly matches the symptom: the link is up, but no trunk appears under show interfaces trunk, and multi-VLAN traffic is not passing.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Both ports are set to dynamic auto, so neither switch initiates DTP negotiation.Correct answer▾
Why this is correct
In dynamic auto mode, a switch port passively waits for DTP negotiation requests. When both ends are dynamic auto, no side initiates the negotiation, so the ports default to access mode. This perfectly matches the symptom: the link is up, but no trunk appears under show interfaces trunk, and multi-VLAN traffic is not passing.
✗There is a native VLAN mismatch between the two switches.Wrong answer — click to see why▾
Why this is wrong here
Confusing DTP trunk negotiation with the operational consequences of a native VLAN mismatch.
✗The connecting cable is a straight-through Ethernet cable rather than a crossover cable.Wrong answer — click to see why▾
Why this is wrong here
Assuming that a crossover cable is mandatory for switch-to-switch connections, ignoring auto-MDIX.
✗One switch lacks a VLAN that exists on the other switch.Wrong answer — click to see why▾
Why this is wrong here
Mistaking the ability to forward traffic for a specific VLAN with the ability to negotiate a trunk.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
Go deeper
Related to this question
Learn chapter
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
DTP
Dynamic Trunking Protocol (DTP) is a Cisco proprietary protocol used to automatically negotiate whether a switch port should operate in access mode or trunk mode.
Key term
Switch
A switch is a networking device that connects devices on a local area network and uses MAC addresses to forward data only to the intended recipient.
About these practice questions
This 200-301 question is part of Courseiva's 1,389-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.