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350-401 Practice Question: Which three statements about QoS trust boundaries…
Which three statements about QoS trust boundaries and marking are true? (Choose three.)
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
✓
The 'mls qos trust cos' command configures the switch to trust the CoS marking on incoming packets.
The trust boundary defines where the device trusts or re-marks QoS markings. Typically, the boundary is at the access layer switch. The 'mls qos trust' command sets trust. By default, Cisco switches do not trust CoS or DSCP; they must be configured. Trust can be extended to IP phones via CDP. Marking can be done at Layer 2 (CoS) or Layer 3 (DSCP).
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
By default, Cisco Catalyst switches trust the CoS value received from connected devices.
Why it's wrong here
Incorrect because by default, Cisco switches do not trust any marking; they set the internal DSCP to 0 unless configured otherwise.
- ✓
The 'mls qos trust cos' command configures the switch to trust the CoS marking on incoming packets.
Why this is correct
Correct because this command sets the trust state to CoS on a switch port.
- ✓
The trust boundary can be extended to an IP phone using CDP, allowing the phone to mark traffic.
Why this is correct
Correct because Cisco IP phones can use CDP to negotiate trust and mark voice traffic with CoS/DSCP.
- ✗
Marking at Layer 2 uses DSCP values in the IP header.
Why it's wrong here
Incorrect because Layer 2 marking uses CoS in the 802.1Q tag, not DSCP which is a Layer 3 field.
- ✓
A switch can re-mark packets by using a policy map with the 'set' command applied to an interface.
Why this is correct
Correct because you can apply a service policy with 'set dscp' or 'set cos' to re-mark traffic on a switch port.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.