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.”
300-410 Control Plane Policing (CoPP) Practice Question
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 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.
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.
CoPP rate-limits control plane traffic in bits per second (bps) by default, but OSPF packets are small and frequent. The rate limit in bps may be insufficient for the packet rate, causing drops of OSPF hello packets. The engineer should use packets per second (pps) for protocol packets like OSPF to avoid this issue.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
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: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
Control Plane Policing (CoPP) — This question tests Control Plane Policing (CoPP) — OSPF neighbours must agree on key parameters..
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. — CoPP rate-limits control plane traffic in bits per second (bps) by default, but OSPF packets are small and frequent. The rate limit in bps may be insufficient for the packet rate, causing drops of OSPF hello packets. The engineer should use packets per second (pps) for protocol packets like OSPF to avoid this issue.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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?
OSPF neighbours must agree on key parameters.
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