- A
The CoPP policy is rate-limiting BGP traffic too aggressively, causing BGP packets to be dropped and the session to flap.
Why wrong: This is incorrect. The ACL error means the CoPP policy does not match BGP traffic at all, so the drops are not from BGP. The police rate is not the cause for BGP flapping.
- B
The ACL in the class-map uses UDP instead of TCP, so it does not match BGP traffic, and the drops are from another protocol.
This is correct. The ACL uses UDP instead of TCP, so it does not match BGP traffic. BGP uses TCP port 179. The drops are from other traffic, and BGP flaps due to lack of protection.
- C
The police rate is set too high, causing the router to drop all BGP traffic.
Why wrong: This is incorrect. The police rate is set to 100 kbps, which is low, but the drops are not from BGP because the ACL does not match BGP. The rate is not the root cause.
- D
The class-map is not applied to the control-plane, so the policy has no effect.
Why wrong: This is incorrect. The class-map is applied to the control-plane via the policy-map, so it is effective for the traffic that matches. The problem is the ACL misconfiguration.
CoPP BGP Flapping
This 300-410 practice question tests your understanding of route maps and route filtering. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. A key principle to apply: coPP (Control Plane Policing). 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 enterprise is using CoPP to protect the control plane. R1 has the following configuration: access-list 100 permit udp any any eq 179 class-map match-any BGP match access-group 100 policy-map COPP class BGP police 100000 20000 conform-action transmit exceed-action drop. Router R2 shows: 'show ip bgp summary' indicates the BGP session to R1 is flapping every 30 seconds. R1's 'show policy-map control-plane' shows drops for class BGP. What is the root cause?
Quick Answer
The answer is that the ACL in the class-map uses UDP instead of TCP, so it does not match BGP traffic, and the drops are from another protocol. This is the root cause because BGP uses TCP port 179 for its session, not UDP; the misconfigured access-list 100 permits UDP traffic, meaning the CoPP policy never actually polices BGP packets, and the drops shown on R1 are from some other UDP-based protocol hitting the same class. On the Cisco CCNP ENARSI 300-410 exam, this question tests your ability to correlate CoPP troubleshooting with BGP flapping—a common trap is to immediately assume the police rate is too low, but the real issue is often a mismatched protocol in the ACL. Remember that BGP is a TCP-based routing protocol, so any CoPP ACL targeting it must specify TCP, not UDP. A quick memory tip: “BGP rides TCP, so CoPP must match TCP 179—UDP drops are a red herring.”
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 ACL in the class-map uses UDP instead of TCP, so it does not match BGP traffic, and the drops are from another protocol.
The root cause is the ACL in the class-map using UDP instead of TCP. BGP uses TCP port 179, so the ACL does not match BGP traffic. The drops shown for class BGP are from other UDP traffic hitting the police rate. The BGP session flaps because it is not protected by CoPP; the misconfigured ACL prevents the policy from matching BGP packets, leaving BGP unprotected. The fix is to change the ACL to permit TCP port 179.
Key principle: CoPP (Control Plane Policing)
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 is rate-limiting BGP traffic too aggressively, causing BGP packets to be dropped and the session to flap.
- ✓
The ACL in the class-map uses UDP instead of TCP, so it does not match BGP traffic, and the drops are from another protocol.
- ✗
The police rate is set too high, causing the router to drop all BGP traffic.
- ✗
The class-map is not applied to the control-plane, so the policy has no effect.
Why it's wrong here
This is incorrect. The class-map is applied to the control-plane via the policy-map, so it is effective for the traffic that matches. The problem is the ACL misconfiguration.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Do not assume that a CoPP policy with drops is directly affecting BGP; check if the ACL matches the correct protocol (TCP for BGP).
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- CoPP (Control Plane Policing)
- BGP uses TCP
- Class-map with ACL
- ACL misconfiguration
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
CoPP (Control Plane Policing)
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review coPP (Control Plane Policing), then practise related 300-410 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Route Maps and Route Filtering — This question tests Route Maps and Route Filtering — CoPP (Control Plane Policing).
What is the correct answer to this question?
The correct answer is: The ACL in the class-map uses UDP instead of TCP, so it does not match BGP traffic, and the drops are from another protocol. — The root cause is the ACL in the class-map using UDP instead of TCP. BGP uses TCP port 179, so the ACL does not match BGP traffic. The drops shown for class BGP are from other UDP traffic hitting the police rate. The BGP session flaps because it is not protected by CoPP; the misconfigured ACL prevents the policy from matching BGP packets, leaving BGP unprotected. The fix is to change the ACL to permit TCP port 179.
What should I do if I get this 300-410 question wrong?
Review coPP (Control Plane Policing), then practise related 300-410 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
CoPP (Control Plane Policing)
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Last reviewed: Jun 18, 2026
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