The answer is that the class-map uses a match statement referencing BGP community values, which are not visible at the QoS classification stage. This is because BGP communities are routing attributes exchanged in BGP updates and stored in the BGP table, but they are never carried in the IP packet header itself. QoS classification in Cisco IOS XR operates exclusively on Layer 2 or Layer 3 header fields—such as DSCP, IP precedence, or CoS—and cannot inspect BGP attributes like community values. On the Cisco SPCOR 350-501 exam, this question tests your understanding of the separation between control plane routing information and data plane packet forwarding. A common trap is assuming that BGP attributes are available for QoS marking, when in fact they are stripped before the packet enters the forwarding path. Memory tip: “Communities stay in the table, not in the packet—QoS needs a header, not a label.”
350-501 Automation and Quality of Service Practice Question
This 350-501 practice question tests your understanding of automation and quality of service. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A network operator deploys a QoS policy on a route reflector to classify traffic based on BGP community values. However, the policy is not matching the intended traffic. Which is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The class-map uses a match statement referencing BGP community values, which are not visible at the QoS classification stage.
BGP community values are exchanged as part of the BGP routing update and are stored in the BGP table, but they are not carried in the IP packet header. QoS classification in Cisco IOS operates on fields within the Layer 2 or Layer 3 packet header (e.g., DSCP, IP precedence, CoS) and cannot inspect BGP attributes like community values. Therefore, a class-map using a match statement for BGP communities will never match traffic at the QoS classification stage, making this the most likely cause of the policy not working.
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 policy-map is applied to the wrong interface.
Why it's wrong here
Interface application affects where the policy is processed, but the match condition itself is invalid.
✓
The class-map uses a match statement referencing BGP community values, which are not visible at the QoS classification stage.
Why this is correct
BGP communities are control-plane attributes; QoS uses packet headers.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The service-policy is applied in the wrong direction (input vs output).
Why it's wrong here
Direction affects when the policy is applied, but the match is still invalid.
✗
The class-default class is consuming all traffic.
Why it's wrong here
Class-default matches unclassified traffic; if a match existed it would work.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between control-plane attributes (like BGP communities) and data-plane packet headers, leading candidates to incorrectly assume that any attribute visible in the routing table can be used for QoS classification.
Detailed technical explanation
How to think about this question
QoS classification in Cisco IOS uses the Modular QoS CLI (MQC) and operates on fields in the IP packet header (e.g., DSCP, IP precedence, TOS) or Layer 2 headers (e.g., CoS, DEI). BGP community values are path attributes carried in BGP update messages and are stored in the BGP routing table (show ip bgp community), but they are not part of the data-plane packet. To classify traffic based on BGP communities, you would need to use a route-map to set a QoS marking (e.g., set ip dscp) on the BGP-learned routes, which then propagates to the FIB and can be matched by QoS policies. This is a common pitfall when operators confuse control-plane attributes with data-plane markings.
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: The class-map uses a match statement referencing BGP community values, which are not visible at the QoS classification stage. — BGP community values are exchanged as part of the BGP routing update and are stored in the BGP table, but they are not carried in the IP packet header. QoS classification in Cisco IOS operates on fields within the Layer 2 or Layer 3 packet header (e.g., DSCP, IP precedence, CoS) and cannot inspect BGP attributes like community values. Therefore, a class-map using a match statement for BGP communities will never match traffic at the QoS classification stage, making this the most likely cause of the policy not working.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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