The answer is that OSPF packets are matched by class-default and rate-limited, causing hello packets to be dropped. This occurs because Control Plane Policing (CoPP) applies a policy-map to the control plane, and the class-default class catches all traffic not explicitly classified into a higher-priority class. When you apply a rate-limit of 10000 pps to class-default, OSPF hello packets—which are not matched by a dedicated class—fall into this default bucket and are subjected to the rate limit. If the limit is too low, hellos are dropped, breaking neighbor adjacencies and triggering CPU spikes from route recalculation. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of CoPP class matching order and the critical need to explicitly classify routing protocol traffic. A common trap is assuming class-default only affects low-priority traffic; in reality, it catches everything not explicitly matched. Memory tip: “Default drops daemons”—if you don’t explicitly protect routing protocols in a CoPP class, the default class will drop them.
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. 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.
A network engineer configures Control Plane Policing (CoPP) on a router with a policy-map that applies a rate-limit of 10000 pps to the class-default class. After applying the policy, the router's CPU utilization spikes and OSPF neighbors go down. What 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
✓
OSPF packets are matched by class-default and rate-limited, causing hello packets to be dropped.
Option A is correct because CoPP applies the policy-map to the control plane, and the class-default class matches all traffic not explicitly classified by other classes. By rate-limiting class-default to 10000 pps, OSPF hello packets (which are not explicitly matched in a higher-priority class) are subjected to the rate limit. If the rate limit is too low, OSPF hellos are dropped, causing neighbor adjacencies to fail and triggering CPU spikes due to route recalculation.
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.
✓
OSPF packets are matched by class-default and rate-limited, causing hello packets to be dropped.
Why this is correct
Without an explicit class for OSPF, it falls into class-default and is subject to the rate-limit, which can drop hellos.
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 rate-limit is in bps, not pps, so it is too low.
Why it's wrong here
The configuration says pps, so it is in packets per second.
✗
The policy-map 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 interfaces.
✗
The router has an ACL that blocks OSPF packets before CoPP processes them.
Why it's wrong here
ACL on interfaces would affect data plane, not control plane.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that class-default is safe to rate-limit aggressively, but the trap here is that critical routing protocol packets (like OSPF hellos) are implicitly matched by class-default unless explicitly classified into a higher-priority class, causing them to be dropped and breaking adjacencies.
Detailed technical explanation
How to think about this question
CoPP uses a hierarchical policy-map where the class-default is a catch-all for unmatched traffic; OSPF packets (protocol 89) are typically placed in a dedicated class for protection, but if omitted, they fall into class-default. The rate-limit of 10000 pps may be insufficient for OSPF hellos (sent every 10 seconds by default) plus other control traffic, causing drops that trigger OSPF dead timer expiration (40 seconds) and subsequent SPF recalculations, which spike CPU. In real-world scenarios, engineers often forget to create a high-priority class for routing protocols, leading to this exact outage.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv4 Access Control Lists — This question tests IPv4 Access Control Lists — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: OSPF packets are matched by class-default and rate-limited, causing hello packets to be dropped. — Option A is correct because CoPP applies the policy-map to the control plane, and the class-default class matches all traffic not explicitly classified by other classes. By rate-limiting class-default to 10000 pps, OSPF hello packets (which are not explicitly matched in a higher-priority class) are subjected to the rate limit. If the rate limit is too low, OSPF hellos are dropped, causing neighbor adjacencies to fail and triggering CPU spikes due to route recalculation.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. CoPP is rate-limiting legitimate routing traffic. Router R1 has: class-map match-any ROUTING, match protocol bgp, match protocol ospf, policy-map COPP, class ROUTING, police 10000 conform-action transmit exceed-action drop. BGP sessions flap. What is the root cause?
hard
A.The class-map should match only BGP, not OSPF.
✓ B.The police rate is too low, causing drops of BGP packets.
C.CoPP should be applied to the control plane, not the data plane.
D.BGP sessions flap due to MTU mismatch, not CoPP.
Why B: B is correct because the police rate of 10,000 bps (10 kbps) is too low for BGP traffic. BGP uses TCP port 179 and can generate bursts of packets during keepalive and update exchanges; if the policer drops BGP packets, the TCP session times out and flaps. The class-map correctly matches both BGP and OSPF, but the rate limit is insufficient for the combined control-plane traffic.
Last reviewed: Jun 24, 2026
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