Question 512 of 2,152

Control Plane Policing (CoPP) →mediumMultiple ChoiceObjective-mapped

CoPP ICMP Rate Limiting — Ping Drops | Cisco CCNP ENARSI 300-410 Explained

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.

A network engineer configures CoPP on a router to limit ICMP traffic to 5000 bps. After the policy is applied, the engineer notices that the router is not responding to ping requests from a remote network. However, the router can ping other devices successfully. The engineer checks the CoPP statistics and sees that the ICMP class has dropped packets. What is the most likely root 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 incoming ICMP echo requests because the police rate is too low.
Incoming ICMP packets are policed by CoPP, and if the rate is exceeded, they are dropped, preventing the router from responding.
B
The CoPP policy is dropping outgoing ICMP echo replies because the police rate applies to both directions.
Why wrong: CoPP applies to incoming traffic to the control plane, not outgoing traffic.
C
The router's interface ACL is blocking incoming ICMP traffic.
Why wrong: The scenario does not mention an ACL; CoPP is the likely cause.
D
The router's ICMP rate-limit feature is enabled globally.
Why wrong: ICMP rate-limit is a separate feature; the scenario focuses on CoPP.

Full breakdown with real-world context →

Quick Answer

The answer is that the CoPP policy is dropping incoming ICMP echo requests because the police rate is too low. CoPP (Control Plane Policing) applies only to traffic destined for the router’s control plane, such as incoming ping requests; when the rate is set to only 5000 bps, it is insufficient to handle the burst of ICMP packets, causing the router to drop them silently and fail to respond. This scenario tests your understanding that CoPP is unidirectional—it polices inbound control plane traffic only, so the router can still originate pings since outgoing ICMP is not affected. On the Cisco CCNP ENARSI 300-410 exam, this is a classic trap where candidates confuse the direction of CoPP enforcement or assume the policy applies symmetrically. A reliable memory tip: “CoPP cops the incoming, not the outgoing”—if the router can ping out but not answer pings in, the police rate is starving the control plane.

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 incoming ICMP echo requests because the police rate is too low.

The CoPP policy is applied to control plane traffic, which includes ICMP echo requests destined to the router itself. When the police rate is set to 5000 bps, incoming ICMP echo requests are rate-limited and dropped if they exceed this threshold, causing the router to stop responding to pings. The fact that the router can still ping other devices confirms that the data plane is unaffected, isolating the issue to control plane policing of inbound ICMP.

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 incoming ICMP echo requests because the police rate is too low.
Why this is correct
Incoming ICMP packets are policed by CoPP, and if the rate is exceeded, they are dropped, preventing the router from responding.
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 CoPP policy is dropping outgoing ICMP echo replies because the police rate applies to both directions.
Why it's wrong here
CoPP applies to incoming traffic to the control plane, not outgoing traffic.
✗
The router's interface ACL is blocking incoming ICMP traffic.
Why it's wrong here
The scenario does not mention an ACL; CoPP is the likely cause.
✗
The router's ICMP rate-limit feature is enabled globally.
Why it's wrong here
ICMP rate-limit is a separate feature; the scenario focuses on CoPP.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates confuse the direction of CoPP policing, assuming it applies to both inbound and outbound control plane traffic, when in fact only inbound traffic to the control plane is policed by default.

Trap categories for this question

Scenario analysis trap
The scenario does not mention an ACL; CoPP is the likely cause.

Detailed technical explanation

How to think about this question

CoPP uses MQC (Modular QoS CLI) to classify and police traffic destined to the control plane via the 'service-policy input' command under 'control-plane'. The police rate of 5000 bps is applied to the aggregate ICMP traffic; if the burst of echo requests exceeds this rate (e.g., from a single ping flood or multiple sources), packets are dropped. In real-world scenarios, this is often misconfigured when the rate is set too low for legitimate monitoring traffic, such as from NMS systems sending frequent pings.

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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.

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 incoming ICMP echo requests because the police rate is too low. — The CoPP policy is applied to control plane traffic, which includes ICMP echo requests destined to the router itself. When the police rate is set to 5000 bps, incoming ICMP echo requests are rate-limited and dropped if they exceed this threshold, causing the router to stop responding to pings. The fact that the router can still ping other devices confirms that the data plane is unaffected, isolating the issue to control plane policing of inbound ICMP.

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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