Question 2,024 of 2,152

Control Plane Policing (CoPP) →hardMultiple ChoiceObjective-mapped

Why CPU Remains High After CoPP with OSPF Class and Low Class-Default Rate

This 300-410 practice question tests your understanding of control plane policing (copp). This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 configures CoPP on a router with the following policy: class-map match-any PROTECT, match protocol ospf, police 1000 pps; class class-default, police 500 pps. After applying, OSPF neighbors form, but the router's CPU utilization remains high. Which is the most likely explanation?

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-default police rate is too low, causing ARP packets to be dropped, but CPU is high due to the policing overhead.
Policing itself consumes CPU, and dropping packets may cause retries, increasing CPU.
B
OSPF traffic is being policed to 1000 pps, which is too high, causing CPU overload.
Why wrong: A higher rate allows more traffic, not less.
C
CoPP only works on hardware-switched platforms, not software.
Why wrong: CoPP works on both, but CPU impact varies.
D
The class-default should have a higher rate than the OSPF class.
Why wrong: There is no such requirement; the issue is the policing overhead.

Full breakdown with real-world context →

Quick Answer

The answer is that the class-default police rate is too low, causing the router to drop essential ARP packets while still consuming CPU cycles due to policing overhead. This occurs because CoPP applies a 1000 pps limit to OSPF traffic, but all other traffic—including ARP, which is critical for neighbor discovery and Layer 2 resolution—is restricted to only 500 pps in the class-default. When ARP requests exceed that low rate, they are dropped, yet the router’s CPU remains high because the policing process itself requires processing cycles to evaluate and drop packets, and OSPF traffic is still being forwarded at its allowed rate. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that CoPP can inadvertently starve control-plane protocols not explicitly matched, and a common trap is assuming high CPU always means too much traffic is passing rather than being policed. Remember the mnemonic: “Low default, high overhead—ARP drops don’t save the CPU.”

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 class-default police rate is too low, causing ARP packets to be dropped, but CPU is high due to the policing overhead.

The correct answer is A because the class-default police rate of 500 pps is too low to handle essential control-plane traffic like ARP, which falls into class-default. When ARP packets are dropped, the router must retry ARP resolution, generating additional CPU overhead from policing and retransmissions, keeping CPU utilization high. OSPF neighbors form because OSPF traffic is explicitly protected in the PROTECT class with a 1000 pps policer, but the underlying ARP starvation causes CPU strain.

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-default police rate is too low, causing ARP packets to be dropped, but CPU is high due to the policing overhead.
Why this is correct
Policing itself consumes CPU, and dropping packets may cause retries, increasing CPU.
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.
✗
OSPF traffic is being policed to 1000 pps, which is too high, causing CPU overload.
Why it's wrong here
A higher rate allows more traffic, not less.
✗
CoPP only works on hardware-switched platforms, not software.
Why it's wrong here
CoPP works on both, but CPU impact varies.
✗
The class-default should have a higher rate than the OSPF class.
Why it's wrong here
There is no such requirement; the issue is the policing overhead.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that CoPP only affects the protected class, but the trap here is that the class-default policer can starve essential control-plane traffic (like ARP) that is not explicitly matched, causing CPU issues even when OSPF appears healthy.

Detailed technical explanation

How to think about this question

Under the hood, CoPP uses a policer that drops packets exceeding the configured rate; ARP packets are typically small and frequent, and a 500 pps limit can easily be exceeded in a network with many neighbors or frequent ARP requests. The router's CPU must process policing decisions and handle ARP retries, which adds overhead beyond the simple packet drop. In real-world scenarios, this is a common misconfiguration where engineers protect routing protocols but forget to allocate sufficient bandwidth for Layer 2 control traffic like ARP, leading to high CPU without complete connectivity loss.

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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.

Visual reference

Quick reference

Routing Protocol Comparison

Protocol	Metric	Max Hops	Algorithm	Type
RIP v2	Hop count	15	Bellman-Ford	Distance vector
OSPF	Cost (bandwidth)	Unlimited	Dijkstra (SPF)	Link state
EIGRP	Composite metric	Unlimited	DUAL	Hybrid
IS-IS	Cost	Unlimited	Dijkstra	Link state
BGP	Policy / attributes	Unlimited	Path vector	Path vector

RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.

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 class-default police rate is too low, causing ARP packets to be dropped, but CPU is high due to the policing overhead. — The correct answer is A because the class-default police rate of 500 pps is too low to handle essential control-plane traffic like ARP, which falls into class-default. When ARP packets are dropped, the router must retry ARP resolution, generating additional CPU overhead from policing and retransmissions, keeping CPU utilization high. OSPF neighbors form because OSPF traffic is explicitly protected in the PROTECT class with a 1000 pps policer, but the underlying ARP starvation causes CPU strain.

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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