The default QoS trust state on a Cisco IOS switch port is untrusted, meaning the port does not preserve any existing Layer 2 Class of Service (CoS) or Layer 3 Differentiated Services Code Point (DSCP) markings on incoming packets. Instead, the switch reclassifies all traffic by marking it with a CoS value of 0, effectively resetting any prior priority tags. This behavior stems from the fact that, without the `mls qos trust` command, the switch assumes the connected device may not be trusted to set accurate QoS markings, so it enforces a default policy of re-marking everything to the lowest priority. On the ENCOR 350-401 exam, this concept often appears in questions about QoS trust boundaries, where a common trap is assuming a port automatically trusts markings from routers or IP phones. To avoid this, remember that trust must be explicitly configured—if you see no `mls qos trust` statement, the port is untrusted and everything gets CoS 0. A helpful memory tip: "No trust, no rust—just CoS 0 dust."
CCNP QoS Practice Question
This 350-401 practice question tests your understanding of qos. 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.
What is the default trust state of a Cisco IOS switch port when no 'mls qos trust' command is configured?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The port is untrusted and marks all incoming packets with CoS 0.
By default, Cisco switches do not trust any QoS markings on incoming packets; they are set to the default CoS/DSCP value of 0 unless a trust policy is applied.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The port trusts the CoS value of incoming packets.
Why it's wrong here
Incorrect. The default is untrusted.
✗
The port trusts the DSCP value of incoming packets.
Why it's wrong here
Incorrect. The default is untrusted.
✓
The port is untrusted and marks all incoming packets with CoS 0.
Why this is correct
Correct. By default, the port is untrusted and packets are marked with CoS 0.
Related concept
Read the scenario before looking for a memorised answer.
✗
The port trusts both CoS and DSCP values.
Why it's wrong here
Incorrect. The default is untrusted.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 350-401 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which 350-401 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
QoS — This question tests QoS — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The port is untrusted and marks all incoming packets with CoS 0. — By default, Cisco switches do not trust any QoS markings on incoming packets; they are set to the default CoS/DSCP value of 0 unless a trust policy is applied.
What should I do if I get this 350-401 question wrong?
Identify which 350-401 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This 350-401 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 350-401 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.