- A
The class-default has an explicit 'drop' action, which overrides the implicit permit and drops all unmatched traffic, including BGP packets.
Explicit deny in class-default changes the default behavior from permit to drop.
- B
The CoPP policy was applied to the wrong interface, so BGP packets are dropped by the interface ACL.
Why wrong: CoPP is applied to the control plane, not interfaces.
- C
The BGP packets are matched by another class with a 'drop' action, but the class-default is irrelevant.
Why wrong: If another class drops BGP, that would be the cause, but the question specifies class-default deny.
- D
The CoPP policy uses 'rate-limit' in bps instead of pps, causing BGP packets to be dropped due to rate limiting.
Why wrong: Rate limiting would cause drops only if exceeded, not a complete failure.
Quick Answer
The answer is that the explicit deny action in class-default overrides the implicit permit, causing CoPP to drop all unmatched traffic, including BGP packets. Control Plane Policing (CoPP) operates with an implicit permit at the end of the class-default by default, meaning any traffic not matched by a higher-priority class is allowed through. However, when an engineer explicitly configures a deny action within class-default, that explicit drop takes precedence over the implicit permit, effectively blackholing all control-plane traffic—including BGP session packets—that isn’t caught by a more specific class. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of CoPP policy evaluation order and the critical distinction between implicit and explicit actions in class-default. A common trap is assuming the implicit permit always applies, but remember: an explicit deny in class-default is absolute. Memory tip: “Explicit beats implicit—if you drop the default, you drop the BGP.”
300-410 BGP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of bgp troubleshooting. 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) with a policy that denies all traffic in class-default. After applying the policy, BGP sessions to the router fail. 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 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 class-default has an explicit 'drop' action, which overrides the implicit permit and drops all unmatched traffic, including BGP packets.
CoPP class-default has an implicit permit at the end, but if an explicit deny is configured in class-default, it will drop all traffic not matched by other classes, including BGP control packets. The explicit deny overrides the implicit permit.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 class-default has an explicit 'drop' action, which overrides the implicit permit and drops all unmatched traffic, including BGP packets.
Why this is correct
Explicit deny in class-default changes the default behavior from permit to drop.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
The CoPP policy was applied to the wrong interface, so BGP packets are dropped by the interface ACL.
Why it's wrong here
CoPP is applied to the control plane, not interfaces.
- ✗
The BGP packets are matched by another class with a 'drop' action, but the class-default is irrelevant.
Why it's wrong here
If another class drops BGP, that would be the cause, but the question specifies class-default deny.
- ✗
The CoPP policy uses 'rate-limit' in bps instead of pps, causing BGP packets to be dropped due to rate limiting.
Why it's wrong here
Rate limiting would cause drops only if exceeded, not a complete failure.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
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BGP Troubleshooting — study guide chapter
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FAQ
Questions learners often ask
What does this 300-410 question test?
BGP Troubleshooting — This question tests BGP Troubleshooting — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The class-default has an explicit 'drop' action, which overrides the implicit permit and drops all unmatched traffic, including BGP packets. — CoPP class-default has an implicit permit at the end, but if an explicit deny is configured in class-default, it will drop all traffic not matched by other classes, including BGP control packets. The explicit deny overrides the implicit permit.
What should I do if I get this 300-410 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 18, 2026
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