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350-401 Practice Question: A network architect is designing QoS for a…
A network architect is designing QoS for a converged network carrying voice, video, and data. The design must use the DiffServ model and ensure that voice traffic is marked with the highest priority and that video traffic is marked with a lower priority but still above data. Which DSCP markings should be assigned to voice and video traffic, respectively, to comply with the standard Per-Hop Behavior (PHB) definitions?
⚠ Common exam trap
Cisco often tests the specific DSCP values for EF (46) and AF41 (34) and the fact that voice must use EF (not AF or CS) to ensure strict priority queuing, while video uses the highest AF class (AF41) to differentiate it from data without breaking the EF 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
✓
Voice: DSCP 46 (EF); Video: DSCP 34 (AF41)
The DiffServ model defines specific Per-Hop Behaviors (PHBs) for different traffic types. Voice traffic requires low latency, jitter, and loss, which is best served by the Expedited Forwarding (EF) PHB, assigned DSCP 46. Video traffic, while still delay-sensitive, can tolerate some loss and is typically marked with Assured Forwarding (AF41, DSCP 34), which provides a lower priority queue than EF but higher than best-effort data.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Voice: DSCP 46 (EF); Video: DSCP 34 (AF41)
Why this is correct
Voice is marked DSCP 46 (EF), which places it in the strict-priority queue in every Cisco router/switch, ensuring minimal delay, jitter, and loss—essential for real-time voice. Video uses DSCP 34 (AF41), an assured-forwarding class with low drop precedence, giving it priority over ordinary data but not over voice, while still allowing it to be dropped gracefully under severe congestion. This pairing follows the standard Cisco Enterprise QoS model and ensures voice quality is never compromised by video bursts.
- ✗
Voice: DSCP 56 (CS7); Video: DSCP 48 (CS6)
Why it's wrong here
DSCP 56 (CS7) and DSCP 48 (CS6) are class-selector codepoints reserved for network control traffic, not user applications. CS6 is the typical marking for routing protocols such as OSPF and BGP, while CS7 is unused but also reserved; using them for voice and video would place user traffic in the same queue as critical routing updates, risking starvation of control packets during congestion. Additionally, these codepoints provide no per-hop behavior like EF or AF—they simply indicate a packet's priority relative to other class-selector values—so they cannot deliver the low-latency treatment voice needs or the bandwidth-assured forwarding video requires.
- ✗
Voice: DSCP 40 (AF41); Video: DSCP 46 (EF)
Why it's wrong here
This option inverts the correct voice and video markings: video gets EF (DSCP 46) and voice gets AF41 (DSCP 34), causing video traffic to be placed in the strict-priority queue. Because EF is serviced before all other queues, bursty video could monopolize link bandwidth and introduce jitter and delay for voice, breaking the primary QoS requirement that voice be the highest-priority application. Voice is latency-sensitive and must always be marked EF to ensure it is queued ahead of video and other data.
- ✗
Voice: DSCP 26 (AF31); Video: DSCP 18 (AF21)
Why it's wrong here
Both DSCP 26 (AF31) and DSCP 18 (AF21) are assured-forwarding codepoints, meaning they are queued in a guaranteed bandwidth class but are never given strict priority. Voice marked as AF31 would compete with other assured-forwarding traffic and could suffer packet loss or delay under congestion, unlike EF which is served first. Video marked as AF21 is even lower in the AF hierarchy (class 2 vs class 3), so it would receive less preferential treatment than voice—yet the real problem is that no AF class can provide the near-zero jitter and delay that voice demands. The correct design assigns EF to voice and a higher AF class (like AF41) to video.
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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.