350-501 Automation and Quality of Service Practice Question
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?
⚠ Common exam trap
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.
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
✓
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, 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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The ACL cannot match on DSCP values.
Why it's wrong here
ACLs can match DSCP.
- ✗
The ACL is applied to the wrong interface.
Why it's wrong 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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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