- A
RTP traffic uses UDP ports, and the class-map must match on the UDP port range instead.
Why wrong: Incorrect because 'match protocol rtp' should match RTP traffic regardless of port, but it requires NBAR.
- B
The 'match protocol rtp' command requires NBAR to be enabled globally with 'ip nbar protocol-discovery'.
Correct because NBAR-based matching requires the 'ip nbar protocol-discovery' command to be enabled on the interface for the classification to work.
- C
The class-map must be configured with 'match any' to capture all traffic.
Why wrong: Incorrect because 'match any' would match all traffic, not specifically RTP.
- D
RTP traffic is always marked with DSCP EF, so the class-map should match on DSCP instead.
Why wrong: Incorrect because RTP traffic may not always be marked, and the engineer wants to match based on protocol, not marking.
Quick Answer
The answer is that the 'match protocol rtp' command requires NBAR to be enabled globally with 'ip nbar protocol-discovery'. This is because NBAR performs deep packet inspection to identify RTP traffic by analyzing payload signatures, and without this global enablement, the router lacks the necessary protocol recognition engine to classify packets using dynamic UDP ports. On the ENCOR 350-401 exam, this scenario tests your understanding that NBAR is not automatically active for match protocol statements—a common trap is assuming the command works out of the box for voice traffic. Remember that RTP uses ephemeral ports, so static ACLs or port-based matching will fail; NBAR’s stateful inspection is mandatory. A useful memory tip: “No NBAR, no RTP bar”—if you forget to enable protocol-discovery globally, your class-map will stay empty.
CCNP QoS Architecture Practice Question
This 350-401 practice question tests your understanding of qos architecture. 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.
An enterprise is deploying a new VoIP system and wants to ensure voice traffic receives priority over data traffic on a WAN link. The engineer configures a class-map to match RTP traffic using the 'match protocol rtp' command. However, the class-map does not match any packets. What is the most likely reason?
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 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 'match protocol rtp' command requires NBAR to be enabled globally with 'ip nbar protocol-discovery'.
The 'match protocol rtp' command relies on Network-Based Application Recognition (NBAR) to identify RTP traffic by inspecting packet payloads and using protocol signatures. Without NBAR enabled globally via 'ip nbar protocol-discovery', the class-map cannot match any packets because the router does not have the necessary deep packet inspection capability. Enabling NBAR allows the device to recognize RTP traffic even though it uses dynamic UDP ports.
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.
- ✗
RTP traffic uses UDP ports, and the class-map must match on the UDP port range instead.
Why it's wrong here
Incorrect because 'match protocol rtp' should match RTP traffic regardless of port, but it requires NBAR.
- ✓
The 'match protocol rtp' command requires NBAR to be enabled globally with 'ip nbar protocol-discovery'.
Why this is correct
Correct because NBAR-based matching requires the 'ip nbar protocol-discovery' command to be enabled on the interface for the classification to work.
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 class-map must be configured with 'match any' to capture all traffic.
Why it's wrong here
Incorrect because 'match any' would match all traffic, not specifically RTP.
- ✗
RTP traffic is always marked with DSCP EF, so the class-map should match on DSCP instead.
Why it's wrong here
Incorrect because RTP traffic may not always be marked, and the engineer wants to match based on protocol, not marking.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume 'match protocol rtp' works out-of-the-box like a simple port match, but Cisco tests the requirement for NBAR to be explicitly enabled for protocol-based matching that relies on deep packet inspection.
Detailed technical explanation
How to think about this question
NBAR uses a protocol discovery engine that examines packets beyond Layer 4 headers, including RTP payload characteristics like the payload type field and synchronization source (SSRC) identifier. Without 'ip nbar protocol-discovery' enabled, the router cannot perform this deep inspection, so the 'match protocol rtp' command has no packets to classify. In real-world deployments, NBAR is often used to classify applications that use dynamic ports, such as VoIP, peer-to-peer, or streaming video, where static port matching is impractical.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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.
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FAQ
Questions learners often ask
What does this 350-401 question test?
QoS Architecture — This question tests QoS Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The 'match protocol rtp' command requires NBAR to be enabled globally with 'ip nbar protocol-discovery'. — The 'match protocol rtp' command relies on Network-Based Application Recognition (NBAR) to identify RTP traffic by inspecting packet payloads and using protocol signatures. Without NBAR enabled globally via 'ip nbar protocol-discovery', the class-map cannot match any packets because the router does not have the necessary deep packet inspection capability. Enabling NBAR allows the device to recognize RTP traffic even though it uses dynamic UDP ports.
What should I do if I get this 350-401 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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Last reviewed: Jun 24, 2026
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.
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