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Control Plane Policing (CoPP) →hardMultiple ChoiceObjective-mapped

CoPP BGP Route Reflector — Missing Prefixes from Updates | 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 CoPP on a router that is a route reflector for iBGP. The policy includes a class-map matching BGP traffic and polices it to 500 pps. After deployment, some iBGP prefixes are missing from the route reflector's table, but the BGP sessions are up. 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
CoPP drops BGP keepalive packets, causing the session to reset.
Why wrong: Sessions are up, so keepalives are not dropped.
B
CoPP drops BGP update packets from specific clients due to rate limiting, so those prefixes are not learned.
Update packets are larger and more frequent; they may exceed the police rate.
C
The route reflector is configured to ignore certain prefixes.
Why wrong: No such configuration is mentioned.
D
CoPP only affects eBGP, not iBGP.
Why wrong: CoPP matches on TCP port 179, which includes both eBGP and iBGP.

Full breakdown with real-world context →

Quick Answer

The answer is that CoPP drops BGP update packets from specific clients due to rate limiting, so those prefixes are not learned. This occurs because the Control Plane Policing policy matches BGP traffic in a class-map and polices it to 500 pps, which can throttle incoming BGP updates from a route reflector client without affecting the session itself. Keepalive packets are smaller and less frequent, so they pass the policer, maintaining the BGP session state, but the larger update packets carrying new prefixes are dropped, leaving the route reflector’s table incomplete. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how CoPP interacts with BGP route reflector operations—a common trap is assuming a missing prefix implies a session reset, when in fact the session stays up because keepalives are not policed. Remember the memory tip: “Keepalives keep the session, but updates update the table—if updates are dropped, prefixes are stopped.”

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

✓

CoPP drops BGP update packets from specific clients due to rate limiting, so those prefixes are not learned.

B is correct because CoPP polices BGP traffic to 500 pps, and if BGP update packets from specific iBGP clients exceed this rate, they are dropped. Since BGP keepalives are small and infrequent, the session remains up, but the dropped updates prevent the route reflector from learning certain prefixes, leading to missing routes in the table.

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.

✗
CoPP drops BGP keepalive packets, causing the session to reset.
Why it's wrong here
Sessions are up, so keepalives are not dropped.
✓
CoPP drops BGP update packets from specific clients due to rate limiting, so those prefixes are not learned.
Why this is correct
Update packets are larger and more frequent; they may exceed the police rate.
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 route reflector is configured to ignore certain prefixes.
Why it's wrong here
No such configuration is mentioned.
✗
CoPP only affects eBGP, not iBGP.
Why it's wrong here
CoPP matches on TCP port 179, which includes both eBGP and iBGP.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that CoPP drops only keepalives or that session state is the sole indicator of BGP health, but the trap here is that BGP sessions can remain up while updates are dropped, leading to missing prefixes without session flaps.

Detailed technical explanation

How to think about this question

CoPP uses a control-plane service policy that applies a policer to matched traffic; in this case, the class-map matches BGP (TCP port 179). BGP updates can be large and bursty, especially when many prefixes are advertised, easily exceeding 500 pps. The route reflector's BGP session stays up because keepalives are small and within the rate, but update packets are dropped, causing incomplete prefix learning—a common issue when CoPP is misconfigured without considering BGP update bursts.

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 practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

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: CoPP drops BGP update packets from specific clients due to rate limiting, so those prefixes are not learned. — B is correct because CoPP polices BGP traffic to 500 pps, and if BGP update packets from specific iBGP clients exceed this rate, they are dropped. Since BGP keepalives are small and infrequent, the session remains up, but the dropped updates prevent the route reflector from learning certain prefixes, leading to missing routes in the table.

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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