Quick Answer
The answer is that the connected device is sending 802.1Q tagged frames. An access port belongs to a single VLAN and expects only untagged traffic, so when it receives tagged frames, it drops them because it lacks the logic to process VLAN tags. This is a core switching concept tested on the Cisco DevNet Associate 200-901 exam, often appearing in troubleshooting scenarios where the port shows up/up but no traffic passes—a classic trap for those who assume link status equals data flow. The key is remembering that access ports strip and ignore tags, while trunk ports handle them. For the exam, think of an access port as a strict bouncer: it only lets in untagged guests, and any frame wearing a tag gets turned away at the door.
This 200-901 practice question tests your understanding of network fundamentals. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An engineer notices that a switch port configured as an access port in VLAN 10 is not forwarding traffic. The switch shows the port is up/up. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Spanning Tree Protocol blocking the port
Why wrong: STP blocking would show in show spanning-tree.
The connected device is sending 802.1Q tagged frames
Access ports drop tagged frames.
Speed/duplex mismatch
Why wrong: Would cause errors or down state.
VLAN 10 does not exist in the VLAN database
Why wrong: VLAN 10 would be created automatically if not present.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The connected device is sending 802.1Q tagged frames
An access port expects to receive and send only untagged frames, as it belongs to a single VLAN (VLAN 10). If the connected device sends 802.1Q tagged frames, the switch will drop them because the access port does not process VLAN tags. This explains why the port is up/up but not forwarding traffic.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Spanning Tree Protocol blocking the port
Why it's wrong here
STP blocking would show in show spanning-tree.
The connected device is sending 802.1Q tagged frames
Why this is correct
Access ports drop tagged frames.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Speed/duplex mismatch
Why it's wrong here
Would cause errors or down state.
VLAN 10 does not exist in the VLAN database
Why it's wrong here
VLAN 10 would be created automatically if not present.
Common exam traps
Cisco often tests the misconception that an access port can handle tagged frames, leading candidates to overlook the strict untagged-only behavior of access ports.
Trap categories for this question
Command / output trap
STP blocking would show in show spanning-tree.
Detailed technical explanation
Access ports strip any 802.1Q tags from incoming frames and forward them as untagged; they also drop any tagged frames received. This behavior is defined by IEEE 802.1Q, where an access port's Port VLAN ID (PVID) is used for untagged traffic. In a real-world scenario, misconfiguring a device (e.g., a VoIP phone or router) to send tagged frames on an access port is a common cause of silent connectivity failures.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Fundamentals — study guide chapter
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FAQ
Network Fundamentals — This question tests Network Fundamentals — Read the scenario before looking for a memorised answer..
The correct answer is: The connected device is sending 802.1Q tagged frames — An access port expects to receive and send only untagged frames, as it belongs to a single VLAN (VLAN 10). If the connected device sends 802.1Q tagged frames, the switch will drop them because the access port does not process VLAN tags. This explains why the port is up/up but not forwarding traffic.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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