Question 1,007 of 2,152

NAT and PAT →hardMultiple ChoiceObjective-mapped

300-410 NAT and PAT Practice Question

This 300-410 practice question tests your understanding of nat and pat. 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 to protect the control plane. After applying the policy, OSPF neighbors go down. The CoPP policy has a class that matches OSPF traffic with a rate-limit of 100 pps. 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 OSPF traffic is being dropped because the rate-limit is too low for the hello interval.
Correct. Low pps rate can drop OSPF hellos, causing neighbor loss.
B
The CoPP policy is applied to the wrong interface.
Why wrong: CoPP is applied globally to control plane, not per interface.
C
The OSPF traffic is matched by the default class and dropped.
Why wrong: If OSPF is explicitly matched, default class does not apply.
D
The CoPP policy uses 'police' instead of 'rate-limit'.
Why wrong: Both can be used; syntax difference does not cause drop.

Full breakdown with real-world context →

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 OSPF traffic is being dropped because the rate-limit is too low for the hello interval.

OSPF uses Hello packets to establish and maintain neighbor adjacencies, typically sent every 10 seconds on broadcast networks. If the CoPP rate-limit of 100 pps is too low to accommodate the burst of OSPF Hello packets (e.g., during neighbor initialization or network convergence), the policer will drop excess packets, causing the OSPF dead timer to expire and neighbors to go down. This is the most direct cause because the policy explicitly matches OSPF traffic and applies a strict rate limit that can starve the control plane of necessary protocol messages.

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 OSPF traffic is being dropped because the rate-limit is too low for the hello interval.
Why this is correct
Correct. Low pps rate can drop OSPF hellos, causing neighbor loss.
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 CoPP policy is applied to the wrong interface.
Why it's wrong here
CoPP is applied globally to control plane, not per interface.
✗
The OSPF traffic is matched by the default class and dropped.
Why it's wrong here
If OSPF is explicitly matched, default class does not apply.
✗
The CoPP policy uses 'police' instead of 'rate-limit'.
Why it's wrong here
Both can be used; syntax difference does not cause drop.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that CoPP is applied per-interface or that the default class is the culprit, but the real trap is understanding that control plane protocols like OSPF require consistent, low-latency delivery of Hello packets, and any rate limit—even one that seems high—can cause neighbor loss if it doesn't account for burst behavior.

Detailed technical explanation

How to think about this question

OSPF Hello packets are sent as multicast to 224.0.0.5, and the default dead interval is 40 seconds (four times the hello interval). If the CoPP policer drops even a few consecutive Hello packets, the neighbor will be declared dead. In real-world scenarios, a rate-limit of 100 pps might seem generous, but if the router is handling many OSPF neighbors or the burst size is large (e.g., during link flapping), the policer's token bucket can empty quickly, causing drops. The 'rate-limit' command uses a single-rate, two-color policer by default, which can drop packets if the average rate exceeds the configured value.

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?

NAT and PAT — This question tests NAT and PAT — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The OSPF traffic is being dropped because the rate-limit is too low for the hello interval. — OSPF uses Hello packets to establish and maintain neighbor adjacencies, typically sent every 10 seconds on broadcast networks. If the CoPP rate-limit of 100 pps is too low to accommodate the burst of OSPF Hello packets (e.g., during neighbor initialization or network convergence), the policer will drop excess packets, causing the OSPF dead timer to expire and neighbors to go down. This is the most direct cause because the policy explicitly matches OSPF traffic and applies a strict rate limit that can starve the control plane of necessary protocol messages.

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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