Question 100 of 2,152

Control Plane Policing (CoPP) →hardMultiple ChoiceObjective-mapped

CoPP OSPF Troubleshooting — Bits per Second vs Packets per Second | Cisco CCNP ENARSI 300-410 Explained

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 Control Plane Policing (CoPP) on a router running OSPF. After applying the policy, OSPF neighbors intermittently drop and recover. The CoPP policy includes a class-map matching OSPF traffic with a police rate of 64000 bps. The router has multiple OSPF neighbors and the link utilization is normal. 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 CoPP policy uses the default class class-default, which drops OSPF packets.
Why wrong: The class-default typically permits traffic by default, not drops it.
B
The police rate is in bits per second, but OSPF hello packets are small; the packet-per-second rate is exceeded, causing drops.
OSPF hello packets are small, so bps rate limiting can be misleading; pps is more appropriate for protocol packets.
C
OSPF uses UDP, and CoPP only filters TCP traffic.
Why wrong: OSPF uses IP protocol 89, not UDP, and CoPP can filter any protocol.
D
The CoPP policy is applied to the wrong interface; it should be applied to the management interface.
Why wrong: CoPP is applied to the control plane, not individual interfaces.

Full breakdown with real-world context →

Quick Answer

The answer is that the police rate configured in bits per second is insufficient because OSPF hello packets are small and frequent, causing the packet-per-second rate to be exceeded and triggering drops. CoPP rate-limits control plane traffic in bps by default, but for protocols like OSPF that send many tiny packets, the bps threshold may be met while the actual packet rate far exceeds what the router can process, leading to intermittent neighbor flaps. On the Cisco CCNP ENARSI 300-410 exam, this question tests your understanding that CoPP police rates default to bps, but protocol packets like OSPF hellos require a pps-based rate to avoid dropping critical keepalives—a common trap where engineers focus only on bandwidth utilization rather than packet frequency. Remember the memory tip: “Small packets, big problem—rate in pps, not bps, for OSPF.”

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 police rate is in bits per second, but OSPF hello packets are small; the packet-per-second rate is exceeded, causing drops.

OSPF hello packets are small (typically 44–64 bytes), so a police rate of 64000 bps translates to a very low packet-per-second (pps) allowance. When multiple OSPF neighbors send hellos and other OSPF packets, the pps limit is easily exceeded, causing CoPP to drop packets and disrupt neighbor adjacencies. This is the most likely cause of intermittent OSPF neighbor drops despite normal link utilization.

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 uses the default class class-default, which drops OSPF packets.
Why it's wrong here
The class-default typically permits traffic by default, not drops it.
✓
The police rate is in bits per second, but OSPF hello packets are small; the packet-per-second rate is exceeded, causing drops.
Why this is correct
OSPF hello packets are small, so bps rate limiting can be misleading; pps is more appropriate for protocol packets.
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 uses UDP, and CoPP only filters TCP traffic.
Why it's wrong here
OSPF uses IP protocol 89, not UDP, and CoPP can filter any protocol.
✗
The CoPP policy is applied to the wrong interface; it should be applied to the management interface.
Why it's wrong here
CoPP is applied to the control plane, not individual interfaces.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that a bps-based police rate is sufficient for small-packet protocols like OSPF, leading candidates to overlook the packet-per-second (pps) limitation that causes drops.

Detailed technical explanation

How to think about this question

CoPP uses a hierarchical policy-map applied to the control plane, and police rates are configured in bps, but the actual rate-limiting is implemented using a token bucket that accounts for both bits and packets. Small packets like OSPF hellos can cause the packet rate to exceed the implicit pps limit derived from the bps rate, especially with many neighbors. In real-world scenarios, engineers should use the 'police rate pps' command or set a higher bps rate to accommodate the pps requirement for routing protocol keepalives.

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 police rate is in bits per second, but OSPF hello packets are small; the packet-per-second rate is exceeded, causing drops. — OSPF hello packets are small (typically 44–64 bytes), so a police rate of 64000 bps translates to a very low packet-per-second (pps) allowance. When multiple OSPF neighbors send hellos and other OSPF packets, the pps limit is easily exceeded, causing CoPP to drop packets and disrupt neighbor adjacencies. This is the most likely cause of intermittent OSPF neighbor drops despite normal link utilization.

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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