The answer is that the interface does not have the 'qos' feature enabled, or the policy-map is not attached. On IOS XR routers, QoS policies require explicit feature enablement via the 'qos' command under the interface configuration; without it, the router compiles the policy successfully but silently ignores it, so traffic is not re-marked from AF11 to AF22. This is a common trap on the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, which tests your understanding that IOS XR treats QoS as a modular feature that must be activated per interface, unlike IOS or IOS-XE where policy attachment alone suffices. The exam often presents a scenario where the policy compiles without errors, leading candidates to overlook the missing 'qos' command. Remember the mnemonic: "QoS on XR? You must declare it there" — always check for the 'qos' keyword under the interface before assuming the marking will work.
350-501 Automation and Quality of Service Practice Question
This 350-501 practice question tests your understanding of automation and quality of service. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
An SP engineer configures a QoS policy on an IOS XR router to re-mark DSCP from AF11 to AF22 for traffic matching a specific ACL. The policy compiles successfully but traffic is not re-marked. What is a likely reason?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface does not have the 'qos' feature enabled, or the policy-map is not attached.
Option C is correct because on IOS XR routers, QoS policies require explicit feature enablement via the 'qos' command under the interface configuration, and the policy-map must be attached in the correct direction. Without the 'qos' feature enabled, the router will compile the policy but silently ignore it, resulting in no re-marking of DSCP from AF11 to AF22.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
If the ACL is applied correctly, traffic would be matched.
✓
The interface does not have the 'qos' feature enabled, or the policy-map is not attached.
Why this is correct
Even with policy-map compiled, without attaching it to the interface, no action occurs.
Related concept
Read the scenario before looking for a memorised answer.
✗
The policy is applied in the output direction; marking can only be done on ingress.
Why it's wrong here
Marking can be done on both directions in IOS XR.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the IOS XR-specific requirement to explicitly enable the 'qos' feature on an interface, which catches candidates accustomed to classic IOS where policy-map attachment alone enables QoS.
Detailed technical explanation
How to think about this question
On IOS XR, the 'qos' feature must be enabled under the interface with the 'qos' command (e.g., 'interface GigabitEthernet0/0/0/0
qos
service-policy input POLICY_NAME') before any QoS policy takes effect. This is a platform-specific requirement that differs from classic IOS, where attaching a policy-map implicitly enables QoS. In real-world deployments, forgetting this step is a common misconfiguration that leads to silent policy failure, often discovered during traffic analysis or DSCP audits.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Automation and Quality of Service — This question tests Automation and Quality of Service — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The interface does not have the 'qos' feature enabled, or the policy-map is not attached. — Option C is correct because on IOS XR routers, QoS policies require explicit feature enablement via the 'qos' command under the interface configuration, and the policy-map must be attached in the correct direction. Without the 'qos' feature enabled, the router will compile the policy but silently ignore it, resulting in no re-marking of DSCP from AF11 to AF22.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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