20+ practice questions focused on QoS Architecture — one of the most tested topics on the ENCOR 350-401 exam. Each question includes a detailed explanation so you learn why the right answer is correct.
Start QoS Architecture PracticeA network engineer is configuring QoS on a Cisco Catalyst 9300 switch to prioritize voice traffic. The switch has multiple access ports connected to IP phones and PCs. The engineer applies a policy-map that matches DSCP EF and sets the CoS to 5. However, after testing, the voice packets are not being marked correctly. What is the most likely cause?
Explanation: On Cisco Catalyst switches like the 9300, QoS marking policies applied via a policy-map only re-mark packets if the interface port is configured to trust a specific marking. Without the 'mls qos trust dscp' command, the switch defaults to an untrusted state and may ignore or overwrite the DSCP-to-CoS mapping set by the policy-map. Option C is correct because the missing trust command prevents the policy-map from correctly applying the CoS 5 marking to voice packets.
An enterprise is deploying a new VoIP system and wants to ensure voice traffic receives priority over data traffic on a WAN link. The engineer configures a class-map to match RTP traffic using the 'match protocol rtp' command. However, the class-map does not match any packets. What is the most likely reason?
Explanation: The 'match protocol rtp' command relies on Network-Based Application Recognition (NBAR) to identify RTP traffic by inspecting packet payloads and using protocol signatures. Without NBAR enabled globally via 'ip nbar protocol-discovery', the class-map cannot match any packets because the router does not have the necessary deep packet inspection capability. Enabling NBAR allows the device to recognize RTP traffic even though it uses dynamic UDP ports.
A network engineer is designing a QoS policy for a Cisco router that connects to an MPLS VPN. The service provider expects all traffic to be marked with IP Precedence values. The engineer wants to ensure that voice traffic (DSCP EF) is mapped to IP Precedence 5. What configuration is required on the router to perform this mapping?
Explanation: Option A is correct because the 'set ip precedence 5' command in a policy-map explicitly marks the IP Precedence field to 5, which corresponds to the same value as DSCP EF (46) in the IP header. This ensures that voice traffic is marked with IP Precedence 5 as required by the service provider, regardless of any existing DSCP markings.
A company is implementing QoS on its campus network. The network engineer configures a policy-map that sets the CoS value for voice traffic to 5 on a switch interface. However, when the traffic reaches the router, the CoS marking is lost. What is the most likely reason?
Explanation: CoS (Class of Service) is a Layer 2 marking field in the 802.1Q VLAN tag, which is stripped when a frame passes through a Layer 3 device (router). Since the router operates at Layer 3, it does not preserve the CoS value; instead, the router must map the CoS to a DSCP (Differentiated Services Code Point) value at Layer 3 to maintain QoS across the routed hop. Option B correctly identifies this fundamental Layer 2 vs. Layer 3 boundary issue.
A network engineer is troubleshooting voice quality issues on a WAN link. The engineer notices that voice packets are being dropped during congestion. The QoS policy uses LLQ for voice traffic, but the priority queue is not providing the expected bandwidth. What is the most likely cause?
Explanation: The priority queue in LLQ uses a built-in policer that drops traffic exceeding the configured bandwidth. When congestion occurs, the policer enforces the bandwidth limit by dropping excess packets, which explains why voice packets are being dropped despite the priority queue being active. This is a fundamental behavior of LLQ to prevent the priority queue from starving other queues.
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Practice all QoS Architecture questions1. Baseline your knowledge
Start with 10 questions to gauge your current understanding of QoS Architecture. This tells you whether you need a concept refresher or just practice.
2. Review every explanation
For each question — right or wrong — read the full explanation. Understanding why an answer is correct is more valuable than knowing the answer itself.
3. Focus on exam traps
QoS Architecture questions on the 350-401 frequently use trap wording. Look for subtle differences in answers that test your precision, not just general knowledge.
4. Reach 80% consistently
Do repeated sessions until you score 80%+ three times in a row. Then move to mixed-mode practice to test cross-topic recall under realistic conditions.
The exact number varies per candidate. QoS Architecture is tested as part of the ENCOR 350-401 blueprint. Practicing with targeted QoS Architecture questions ensures you can handle any format or difficulty that appears.
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Difficulty is subjective, but QoS Architecture is a high-priority exam concept tested in multiple ways — direct recall, scenario analysis, and command-output interpretation. Consistent practice is the best way to build confidence.
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