Question 422 of 2,152

Control Plane Policing (CoPP) →hardMultiple ChoiceObjective-mapped

Troubleshooting CoPP Class-Map with Zero Matches

This 300-410 practice question tests your understanding of control plane policing (copp). Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 runs the following command on Router R1:

R1# show access-lists 100

Extended IP access list 100

10 permit icmp any any echo
    
20 permit icmp any any echo-reply
    
30 permit icmp any any time-exceeded
    
40 permit icmp any any unreachable

R1# show policy-map control-plane

Control Plane

Service-policy input: CoPP-IN

Class-map: CoPP-ICMP (match-all) 0 packets, 0 bytes 5 minute offered rate 0000 bps, drop rate 0000 bps Match: access-group 100 police: cir 8000 bps, bc 1500 bytes, be 1500 bytes conformed 0 packets, 0 bytes; actions: transmit exceeded 0 packets, 0 bytes; actions: drop violated 0 packets, 0 bytes; actions: drop

Based on this output, what is the most likely problem?

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 class-map uses 'match-all' instead of 'match-any'.
Why wrong: With a single match criterion, 'match-all' and 'match-any' behave the same.
B
The access-list does not include all ICMP types that may be sent to the control plane.
The access-list only matches specific ICMP types, potentially missing others like router advertisement or parameter problem.
C
The police rate is too low and is dropping all packets.
Why wrong: The counters show 0 packets, so no packets have been classified to be dropped.
D
The policy-map is applied to the wrong direction.
Why wrong: The policy is applied to input, which is correct for CoPP.

Full breakdown with real-world context →

Quick Answer

The answer is that the class-map CoPP-ICMP shows zero packets matched because the referenced access-list 100 does not account for all ICMP types the control plane actually receives. This occurs because the access-list only permits specific ICMP types—echo, echo-reply, time-exceeded, and unreachable—but the router’s control plane may be processing other ICMP types, such as parameter-problem or redirect, which are not listed and thus get dropped before the class-map can count them. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how Control Plane Policing (CoPP) relies on precise ACL entries to classify traffic; a common trap is assuming a few common ICMP types are sufficient, when in reality the control plane often handles a broader set. Remember that CoPP class-map with zero matches usually points to an incomplete match criterion, not a misapplied policy. A useful memory tip is “ICMP types are like keys—missing one locks the door,” so always verify you’ve covered all types the router might need to police.

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 access-list does not include all ICMP types that may be sent to the control plane.

Option B is correct because the access list 100 only permits four specific ICMP types (echo, echo-reply, time-exceeded, unreachable), but the control plane may receive many other ICMP types (e.g., parameter problem, redirect, timestamp, address mask request) that are not explicitly permitted. Since the class-map CoPP-ICMP matches all traffic that hits access-group 100, any ICMP packet with a type not listed will not match the class and will be handled by the default control-plane policy, which typically drops or polices such traffic, leading to legitimate ICMP packets being dropped.

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 class-map uses 'match-all' instead of 'match-any'.
Why it's wrong here
With a single match criterion, 'match-all' and 'match-any' behave the same.
✓
The access-list does not include all ICMP types that may be sent to the control plane.
Why this is correct
The access-list only matches specific ICMP types, potentially missing others like router advertisement or parameter problem.
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 police rate is too low and is dropping all packets.
Why it's wrong here
The counters show 0 packets, so no packets have been classified to be dropped.
✗
The policy-map is applied to the wrong direction.
Why it's wrong here
The policy is applied to input, which is correct for CoPP.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that a permit ACL in a CoPP class-map automatically allows all ICMP traffic, when in fact only the explicitly listed ICMP types are permitted, and any unlisted types are implicitly denied by the class-map and handled by the default policy.

Trap categories for this question

Command / output trap
The counters show 0 packets, so no packets have been classified to be dropped.

Detailed technical explanation

How to think about this question

Control Plane Policing (CoPP) uses MQC (Modular QoS CLI) to classify and police traffic destined to the route processor. The access list in this scenario acts as a permit filter for the class-map; only traffic matching a permit entry is considered part of the class and subject to the police action. ICMP types are defined in RFC 792, and common control-plane ICMP types include echo (8), echo-reply (0), time-exceeded (11), unreachable (3), parameter problem (12), redirect (5), timestamp (13), and address mask request (17). Missing types like parameter problem can cause PMTUD failures or routing issues in real-world networks.

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.

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 access-list does not include all ICMP types that may be sent to the control plane. — Option B is correct because the access list 100 only permits four specific ICMP types (echo, echo-reply, time-exceeded, unreachable), but the control plane may receive many other ICMP types (e.g., parameter problem, redirect, timestamp, address mask request) that are not explicitly permitted. Since the class-map CoPP-ICMP matches all traffic that hits access-group 100, any ICMP packet with a type not listed will not match the class and will be handled by the default control-plane policy, which typically drops or polices such traffic, leading to legitimate ICMP packets being dropped.

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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