Question 77 of 2,152

Control Plane Policing (CoPP) →easyMultiple ChoiceObjective-mapped

CoPP Rate-Limiting NTP Traffic

This 300-410 practice question tests your understanding of control plane policing (copp). 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 engineer applies a CoPP policy to a router to protect the control plane from a DDoS attack. The policy includes a class-map matching UDP traffic to port 123 (NTP) and polices it to 1000 bps. After the policy is applied, the engineer notices that the router's clock is not synchronizing with its NTP server. The NTP server is reachable via ping. What 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.

A
The CoPP policy is dropping NTP packets because the police rate is too low.
NTP packets are small, but if the police rate is too low, they can be dropped, causing synchronization failure.
B
The NTP server is not responding because of the DDoS attack.
Why wrong: The scenario says the NTP server is reachable via ping, so it is responding.
C
The CoPP class-map is not matching NTP packets because it uses the wrong port number.
Why wrong: The scenario states it matches port 123, so it should match.
D
The router's NTP configuration has a wrong server IP address.
Why wrong: The scenario does not indicate a configuration error; the issue is CoPP-related.

Full breakdown with real-world context →

Quick Answer

The answer is that the CoPP policy is dropping NTP packets because the police rate of 1000 bps is too low. NTP relies on UDP port 123, and even a modest exchange of timestamp packets can easily exceed a 1000 bps limit, especially when considering packet headers and bursty synchronization requests. This rate-limiting causes the router to discard NTP traffic, preventing clock synchronization even though the NTP server remains reachable via ping—a key clue that the issue is not connectivity but a control-plane policing restriction. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that CoPP policies must account for the actual bandwidth of critical protocols; a common trap is assuming ping success means all traffic is passing. Remember: NTP is chatty, so a 1000 bps police rate is like a straw for a firehose—always verify your CoPP class-map thresholds against the protocol’s real-world packet size and frequency.

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 CoPP policy is dropping NTP packets because the police rate is too low.

The CoPP policy is policing NTP traffic (UDP port 123) to only 1000 bps. NTP synchronization requires a steady exchange of packets, and a 1000 bps rate is extremely low—likely insufficient to allow the NTP packets through, causing them to be dropped. Since the NTP server is reachable via ping (ICMP is not affected by this CoPP policy), the issue is clearly that the police rate is too restrictive for NTP traffic.

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 CoPP policy is dropping NTP packets because the police rate is too low.
Why this is correct
NTP packets are small, but if the police rate is too low, they can be dropped, causing synchronization failure.
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 NTP server is not responding because of the DDoS attack.
Why it's wrong here
The scenario says the NTP server is reachable via ping, so it is responding.
✗
The CoPP class-map is not matching NTP packets because it uses the wrong port number.
Why it's wrong here
The scenario states it matches port 123, so it should match.
✗
The router's NTP configuration has a wrong server IP address.
Why it's wrong here
The scenario does not indicate a configuration error; the issue is CoPP-related.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that a CoPP policy will only drop malicious traffic, but the trap here is that an overly restrictive police rate can inadvertently drop legitimate control-plane traffic like NTP, even when the class-map and port numbers are correctly configured.

Trap categories for this question

Scenario analysis trap
The scenario says the NTP server is reachable via ping, so it is responding.

Detailed technical explanation

How to think about this question

CoPP uses a policy-map applied to the control-plane with a class-map that matches specific traffic (e.g., NTP on UDP 123) and then polices it using a conform-action (transmit) and exceed-action (drop). NTP synchronization relies on multiple packet exchanges (client requests and server replies) and can be disrupted if even a small percentage of packets are dropped. In real-world scenarios, a police rate of 1000 bps (about 1-2 NTP packets per second) is often too low for reliable synchronization, especially if the NTP server uses burst or iburst options.

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 300-410 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.

Visual reference

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 300-410 question test?

Control Plane Policing (CoPP) — This question tests Control Plane Policing (CoPP) — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The CoPP policy is dropping NTP packets because the police rate is too low. — The CoPP policy is policing NTP traffic (UDP port 123) to only 1000 bps. NTP synchronization requires a steady exchange of packets, and a 1000 bps rate is extremely low—likely insufficient to allow the NTP packets through, causing them to be dropped. Since the NTP server is reachable via ping (ICMP is not affected by this CoPP policy), the issue is clearly that the police rate is too restrictive for NTP traffic.

What should I do if I get this 300-410 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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