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350-401 Practice Question: The default CoS-to-queue mapping on a Cisco…
What is the default CoS-to-queue mapping on a Cisco switch that supports QoS?
⚠ Common exam trap
Cisco often tests the default CoS-to-queue mapping as a memorization point, and the trap here is that candidates confuse the default mapping with a custom or logical grouping, such as assuming CoS 5 is always in the highest queue or that each CoS gets its own queue.
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
✓
CoS 0-1 to queue 1, CoS 2-3 to queue 2, CoS 4-5 to queue 3, CoS 6-7 to queue 4
On Cisco switches that support QoS, the default Class of Service (CoS) to queue mapping distributes CoS values across four egress queues. CoS 0 and 1 are mapped to queue 1 (best effort), CoS 2 and 3 to queue 2, CoS 4 and 5 to queue 3, and CoS 6 and 7 to queue 4 (highest priority). This mapping is defined by the default trust state and is used to prioritize traffic based on the 802.1p priority bits in the VLAN tag.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
CoS 0-1 to queue 1, CoS 2-3 to queue 2, CoS 4-5 to queue 3, CoS 6-7 to queue 4
Why this is correct
This is the default CoS-to-queue assignment on many Cisco Catalyst switches, where the eight CoS priorities are paired: CoS 0-1 map to queue 1, CoS 2-3 to queue 2, CoS 4-5 to queue 3, and CoS 6-7 to queue 4. This pairing reflects the hardware's four egress queues and allows differentiated treatment for traffic classes while preserving the eight CoS fields. In this default configuration, queue scheduling (typically SRR) uses these pairs to allocate bandwidth without requiring custom QoS policy.
- ✗
CoS 0-2 to queue 1, CoS 3-5 to queue 2, CoS 6-7 to queue 3
Why it's wrong here
This grouping is incorrect as a default mapping because it uses only three egress queues and unevenly distributes CoS values (0-2, 3-5, 6-7). Cisco's default four-queue scheme never groups three CoS values into one queue; it consistently maps two consecutive CoS values per queue. A user might configure such a custom map via mls qos srr-queue, but it does not represent the out-of-box behavior.
- ✗
CoS 0 to queue 1, CoS 1 to queue 2, CoS 2 to queue 3, CoS 3 to queue 4
Why it's wrong here
This option maps only CoS 0 through 3 and leaves CoS 4-7 undefined, which would cause those higher priorities to fall back to a default queue or require additional configuration. It also assigns a single CoS to each queue, which is unnecessarily granular compared to the default pairing (0-1, 2-3, etc.) and would force separate queues for adjacent priorities. Default QoS architectures on Cisco switches intentionally group two CoS values per queue to balance scheduling granularity and hardware queue availability.
- ✗
All CoS values are mapped to a single queue by default.
Why it's wrong here
Mapping all CoS values to a single queue would negate the purpose of QoS classification and differentiation. Even with QoS disabled or in 'trust CoS' default mode, Cisco switches allocate traffic across multiple egress queues based on the 802.1p priority (typically four queues). A single-queue design would require explicit configuration of a custom policy or an unusual hardware mode and is not the default QoS architecture.
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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.