The correct answer is that only the SHAKE-1M policy is applied, and the CHILD-QOS policy is ignored. This occurs because Cisco IOS XR enforces a strict service-policy limitation of only one policy per interface direction; when a second policy is applied to the same subinterface in the outbound direction, the router honors only the first policy applied and silently discards the second. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this concept tests your understanding of QoS hierarchy and interface-level constraints, often appearing as a trap where you might assume policies stack or merge. A common mistake is thinking multiple policies can be chained, but IOS XR does not allow that—each direction (inbound or outbound) supports exactly one active policy-map. Memory tip: think “first applied, last standing”—the router remembers only the initial service-policy in a given direction, so always verify the order of application when troubleshooting QoS on subinterfaces.
350-501 Automation and Quality of Service Practice Question
This 350-501 practice question tests your understanding of automation and quality of service. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.
Exhibit
Refer to the exhibit.
policy-map SHAPE-1M
class class-default
shape average 1000000
!
policy-map CHILD-QOS
class VOICE
priority level 1
police cir 256000
class DATA
bandwidth remaining ratio 50
class MANAGEMENT
bandwidth remaining ratio 10
!
interface GigabitEthernet0/0/0/0.100
service-policy output SHAPE-1M
service-policy output CHILD-QOS
Refer to the exhibit. An engineer applies two service policies to the same subinterface in the outbound direction. Which statement describes the expected behavior?
Refer to the exhibit.
policy-map SHAPE-1M
class class-default
shape average 1000000
!
policy-map CHILD-QOS
class VOICE
priority level 1
police cir 256000
class DATA
bandwidth remaining ratio 50
class MANAGEMENT
bandwidth remaining ratio 10
!
interface GigabitEthernet0/0/0/0.100
service-policy output SHAPE-1M
service-policy output CHILD-QOS
A
The router automatically nests CHILD-QOS inside SHAKE-1M creating hierarchical QoS.
Why wrong: Hierarchical QoS must be explicitly configured using a parent-child service-policy.
B
The router compiles a combined policy that merges classifications.
Why wrong: No merging; only one policy is active.
C
Only the SHAKE-1M policy is applied; the CHILD-QOS policy is ignored.
Correct: Only one output policy allowed per interface on IOS XR.
D
Both policies are applied sequentially; traffic is shaped then classified.
Why wrong: Only the first policy applied takes effect.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Only the SHAKE-1M policy is applied; the CHILD-QOS policy is ignored.
When two service policies are applied to the same subinterface in the same direction, the router only honors the first policy applied; the second policy is ignored. In this case, SHAKE-1M was applied first, so CHILD-QOS is not processed. Cisco IOS does not allow multiple service policies in the same direction on a single interface or subinterface, as each direction can have only one active policy-map.
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.
✗
The router automatically nests CHILD-QOS inside SHAKE-1M creating hierarchical QoS.
Why it's wrong here
Hierarchical QoS must be explicitly configured using a parent-child service-policy.
✗
The router compiles a combined policy that merges classifications.
Why it's wrong here
No merging; only one policy is active.
✓
Only the SHAKE-1M policy is applied; the CHILD-QOS policy is ignored.
Why this is correct
Correct: Only one output policy allowed per interface on IOS XR.
Related concept
Read the scenario before looking for a memorised answer.
✗
Both policies are applied sequentially; traffic is shaped then classified.
Why it's wrong here
Only the first policy applied takes effect.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume multiple service policies can be stacked or merged in the same direction, but Cisco enforces a strict one-policy-per-direction rule, making the second policy silently ignored.
Detailed technical explanation
How to think about this question
Under the hood, Cisco IOS maintains a single policy-map per direction per interface; attempting to attach a second policy-map in the same direction results in an error or silent rejection, depending on the platform. This behavior is enforced by the QoS infrastructure, which uses a single token bucket or shaping mechanism per direction. In real-world scenarios, to combine shaping and queuing, engineers must use a hierarchical policy where the parent policy shapes and the child policy applies queuing or marking within a class.
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 practitioner preparing for the 350-501 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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: Only the SHAKE-1M policy is applied; the CHILD-QOS policy is ignored. — When two service policies are applied to the same subinterface in the same direction, the router only honors the first policy applied; the second policy is ignored. In this case, SHAKE-1M was applied first, so CHILD-QOS is not processed. Cisco IOS does not allow multiple service policies in the same direction on a single interface or subinterface, as each direction can have only one active policy-map.
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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