350-401 Infrastructure Practice Question
A network engineer is implementing QoS on a Cisco IOS router. The requirement is to classify traffic based on the DSCP value in the IP header and then mark it with a new DSCP value. Which QoS mechanism should be used to accomplish this?
⚠ Common exam trap
Candidates often confuse QoS mechanisms that use MQC (like CBWFQ and LLQ) with MQC itself. CBWFQ and LLQ are queuing actions within a policy-map, but the classification and marking is done by the MQC framework.
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
✓
Modular QoS CLI (MQC)
The Modular QoS CLI (MQC) is the correct framework for classifying traffic based on DSCP and marking it with a new DSCP value. It involves creating a class-map to match the desired DSCP, a policy-map to set the new DSCP, and applying the service-policy to an interface. This provides a flexible and standardized way to implement QoS policies.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Class-based weighted fair queueing (CBWFQ)
Why it's wrong here
CBWFQ is a queuing mechanism that provides guaranteed bandwidth to classes of traffic. It is configured using MQC, but CBWFQ itself is about queueing and bandwidth allocation, not about classification and marking. While you can use MQC to configure CBWFQ, the question specifically asks for classification and marking, which is done by MQC class-maps and policy-maps.
- ✗
Policy-based routing (PBR)
Why it's wrong here
PBR is used to forward packets based on policy rather than the routing table. It can match on IP precedence or DSCP, but its primary function is to set the next-hop or interface, not to remark DSCP. While PBR can include set commands for DSCP, it is not the dedicated mechanism for classification and marking. The modular QoS CLI (MQC) is the correct tool.
- ✗
Low latency queueing (LLQ)
Why it's wrong here
LLQ is a queuing mechanism that provides strict priority to delay-sensitive traffic like voice. It is configured within MQC using the priority command in a policy-map. LLQ does not perform classification or marking; it only schedules packets. Therefore, it is not the correct mechanism for the stated requirement.
- ✓
Modular QoS CLI (MQC)
Why this is correct
MQC is the framework for configuring QoS on Cisco IOS. It uses class-maps to classify traffic (e.g., matching DSCP) and policy-maps to define actions such as marking with a new DSCP value. The service-policy command applies the policy to an interface. This is the standard and correct method to classify and mark traffic based on DSCP.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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