Question 1,538 of 2,152

Control Plane Policing (CoPP) →hardMultiple ChoiceObjective-mapped

Control Plane Policing: Why High-Latency BGP Peers Flap

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.

A router has CoPP configured with a class-map that matches BGP traffic (TCP port 179) and polices it to 500 pps. The router has multiple iBGP peers. After applying the policy, some BGP sessions flap, but others remain stable. The flapping peers are those with higher latency. 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 packets based on source IP, and high-latency peers have different source IPs.
Why wrong: CoPP matches on protocol, not source IP, unless specifically configured.
B
High-latency peers generate more TCP retransmissions, which are more likely to be dropped by the police rate, causing session flaps.
TCP retransmissions increase with latency, and CoPP may drop them, leading to BGP session failure.
C
BGP uses UDP for keepalives, and CoPP only polices TCP.
Why wrong: BGP uses TCP for all communications.
D
The CoPP policy is applied to the wrong control plane; it should be applied to the forwarding plane.
Why wrong: CoPP is applied to the control plane.

Full breakdown with real-world context →

Quick Answer

The answer is that high-latency BGP peers generate more TCP retransmissions, which are more likely to be dropped by the CoPP police rate, causing session flaps. When a BGP peer has higher latency, TCP windowing forces more frequent retransmissions of keepalives and updates to maintain the session; these extra packets exceed the 500 pps police rate and get dropped, while lower-latency peers stay within the limit and remain stable. This question tests your understanding of how CoPP interacts with TCP behavior under latency, a common trap on the Cisco CCNP ENARSI 300-410 exam where candidates mistakenly blame routing protocol timers instead of TCP retransmission dynamics. The key insight is that CoPP does not differentiate between original packets and retransmissions, so any burst of TCP traffic from a slow link can push the session over the policer threshold. Memory tip: “Latency leads to retransmission, retransmission leads to drops, drops lead to flaps—CoPP punishes the slowest link first.”

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

✓

High-latency peers generate more TCP retransmissions, which are more likely to be dropped by the police rate, causing session flaps.

B is correct because high-latency BGP peers experience more TCP retransmissions due to delayed acknowledgments. The CoPP policer drops packets exceeding 500 pps, and these retransmissions increase the packet rate for those sessions, making them more likely to exceed the policer and be dropped. Dropping BGP TCP segments (including keepalives) causes the BGP hold timer to expire, leading to session flaps.

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 packets based on source IP, and high-latency peers have different source IPs.
Why it's wrong here
CoPP matches on protocol, not source IP, unless specifically configured.
✓
High-latency peers generate more TCP retransmissions, which are more likely to be dropped by the police rate, causing session flaps.
Why this is correct
TCP retransmissions increase with latency, and CoPP may drop them, leading to BGP session failure.
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.
✗
BGP uses UDP for keepalives, and CoPP only polices TCP.
Why it's wrong here
BGP uses TCP for all communications.
✗
The CoPP policy is applied to the wrong control plane; it should be applied to the forwarding plane.
Why it's wrong here
CoPP is applied to the control plane.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that CoPP drops packets based on source IP or that BGP uses UDP for keepalives, leading candidates to overlook the impact of TCP retransmissions from high-latency peers on policer thresholds.

Detailed technical explanation

How to think about this question

TCP retransmissions occur when the sender does not receive an ACK within the retransmission timeout (RTO), which is influenced by round-trip time (RTT). For high-latency peers, the RTO is longer, but the number of retransmissions can spike due to packet loss or delayed ACKs, increasing the packet-per-second rate for that session. CoPP uses a token-bucket policer; exceeding the 500 pps rate causes tail drop, which can selectively impact sessions with higher retransmission rates, while stable peers stay within the policed rate.

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: High-latency peers generate more TCP retransmissions, which are more likely to be dropped by the police rate, causing session flaps. — B is correct because high-latency BGP peers experience more TCP retransmissions due to delayed acknowledgments. The CoPP policer drops packets exceeding 500 pps, and these retransmissions increase the packet rate for those sessions, making them more likely to exceed the policer and be dropped. Dropping BGP TCP segments (including keepalives) causes the BGP hold timer to expire, leading to session flaps.

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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