CCNP Infrastructure Practice Question
A network engineer is configuring QoS on a Cisco IOS XE router. The router must mark all ingress traffic from a specific subnet with DSCP AF31 and ensure that this marking is trusted throughout the network. Which configuration step is required to achieve this?
⚠ Common exam trap
It's easy for candidates to confuse QoS marking with trusting markings; trusting DSCP only preserves existing values and does not mark unmarked traffic.
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
✓
Create a class map matching the subnet, then a policy map that sets DSCP AF31, and apply it as a service policy on the ingress interface.
To mark traffic from a specific subnet with DSCP AF31, you need a class map to identify the traffic, a policy map to set the DSCP, and the policy applied as an ingress service policy. This ensures packets are marked before any queuing or forwarding decisions, allowing downstream devices to trust and act on the marking.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Apply a policy map that sets DSCP AF31 as an output service policy on the router's uplink interface.
Why it's wrong here
Applying the marking policy on the uplink egress would mark traffic after it has already traversed the router's internal queuing and potentially other interfaces. Marking should occur as close to the source as possible, typically at ingress, to ensure consistent treatment throughout the network. Egress marking on the uplink may also affect traffic from other sources.
- ✓
Create a class map matching the subnet, then a policy map that sets DSCP AF31, and apply it as a service policy on the ingress interface.
Why this is correct
To mark traffic from a subnet, you must classify it with a class map, define the marking action in a policy map using 'set dscp af31', and attach the policy to the ingress interface with 'service-policy input'. This ensures packets are marked at ingress, and subsequent devices can trust the DSCP if configured to do so.
- ✗
Configure 'mls qos trust dscp' on the ingress interface and rely on the subnet's existing markings.
Why it's wrong here
Trusting DSCP only preserves existing markings; it does not mark traffic that arrives unmarked. The scenario requires marking all traffic from a specific subnet, which implies the traffic may not already have the desired DSCP. Therefore, trusting alone is insufficient; you must actively set the DSCP value.
- ✗
Use a route map to set DSCP AF31 for all routes matching the subnet and redistribute into OSPF.
Why it's wrong here
Route maps are used for routing policy, such as redistribution or path selection, and cannot mark data plane traffic with DSCP. DSCP marking requires QoS policy maps applied to interfaces. Route maps operate on routing updates, not on individual packets. This approach would not affect the DSCP of transit traffic.
Visual reference
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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
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.