Quick Answer
The answer is CoS 5 at Layer 2 and DSCP EF at Layer 3. This combination is correct because voice traffic requires strict priority queuing to ensure low latency and jitter, and CoS 5 maps directly to the priority queue on Cisco switches while DSCP EF (value 46) provides the standardized Expedited Forwarding per-hop behavior defined in RFC 3246. On the ENCOR 350-401 exam, this pairing tests your understanding of end-to-end QoS marking across the OSI layers, often appearing in questions about campus network access ports where both Layer 2 and Layer 3 markings must be trusted. A common trap is confusing CoS 5 with CoS 3 or DSCP AF41, which are used for data or video, not voice. Remember the memory tip: “Voice is 5 and 46” — CoS 5 for Layer 2, DSCP 46 (EF) for Layer 3, ensuring voice traffic always gets the fast lane.
This 350-401 practice question tests your understanding of infrastructure. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
A company is implementing QoS in a campus network. Voice traffic must be prioritized over data traffic, and all traffic should be marked at Layer 2 and Layer 3. Which combination of marking values should be used on access ports to achieve this?
CoS 5, DSCP AF41
Why wrong: AF41 is used for video, not voice.
CoS 5, DSCP CS3
Why wrong: CS3 is lower priority than EF.
CoS 5, DSCP EF
CoS 5 and DSCP EF are the standard marks for voice.
CoS 4, DSCP EF
Why wrong: CoS 4 is for video conferencing.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
CoS 5, DSCP EF
Option C is correct because voice traffic requires strict priority queuing, which is achieved by marking with CoS 5 at Layer 2 and DSCP EF (46) at Layer 3. CoS 5 maps to the priority queue in Cisco switches, and DSCP EF is the standard per-hop behavior for Expedited Forwarding (RFC 3246), ensuring low latency and jitter for voice. Access ports must trust these markings to prioritize voice over data 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.
CoS 5, DSCP AF41
Why it's wrong here
AF41 is used for video, not voice.
CoS 5, DSCP CS3
Why it's wrong here
CS3 is lower priority than EF.
CoS 5, DSCP EF
CoS 4, DSCP EF
Why it's wrong here
CoS 4 is for video conferencing.
Common exam traps
The trap here is that candidates confuse CoS 5 with DSCP EF for voice but may pick CoS 4 (used for video) or DSCP AF41 (used for premium data), failing to recognize that voice requires both strict priority marking (CoS 5) and the Expedited Forwarding PHB (DSCP EF) to guarantee low-latency treatment.
Detailed technical explanation
Under the hood, Cisco switches use the CoS-to-queue mapping; by default, CoS 5 maps to queue 4 (the priority queue) on most Catalyst platforms, while DSCP EF is mapped to the same queue via the MQC or auto-QoS. In a real-world scenario, if an access port is configured with 'mls qos trust cos' or 'trust dscp', the switch will honor the markings; mismatched CoS and DSCP (e.g., CoS 5 with DSCP AF41) could cause the switch to use only one marking, potentially dropping voice packets during congestion. Auto-QoS for VoIP simplifies this by setting CoS 5 and DSCP EF on the phone and trusting the marking on the switch port.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Infrastructure — This question tests Infrastructure — Read the scenario before looking for a memorised answer..
The correct answer is: CoS 5, DSCP EF — Option C is correct because voice traffic requires strict priority queuing, which is achieved by marking with CoS 5 at Layer 2 and DSCP EF (46) at Layer 3. CoS 5 maps to the priority queue in Cisco switches, and DSCP EF is the standard per-hop behavior for Expedited Forwarding (RFC 3246), ensuring low latency and jitter for voice. Access ports must trust these markings to prioritize voice over data traffic.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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