- A
The class-default policy is dropping BGP packets that are not matched by the BGP class due to an incomplete ACL.
Correct. If the ACL does not match all BGP packets (e.g., only matching TCP port 179 from one direction), some BGP packets fall into class-default and are dropped.
- B
The police rate for BGP is too low, causing BGP packets to be dropped even when matched.
Why wrong: Incorrect. The BGP police rate is 100 pps, which is typically sufficient for BGP keepalives and updates. The flapping is due to dropped packets from class-default.
- C
The CoPP policy is applied to the wrong control-plane (e.g., 'control-plane host' instead of 'control-plane transit').
Why wrong: Incorrect. Applying to the wrong control-plane would affect different traffic, but the symptom would still be BGP flapping if the policy were applied to host.
- D
The 'service-policy' command is missing the 'input' keyword, causing the policy to be applied only to output traffic.
Why wrong: Incorrect. The 'input' keyword is required for CoPP, but if it were missing, the policy would not be applied at all, and BGP would not flap.
Quick Answer
The answer is an incomplete ACL causing BGP packets to fall into the class-default and be dropped. CoPP processes packets sequentially through its policy-map classes; if the ACL in the BGP class does not match all BGP traffic—such as missing the return path for TCP port 179 or failing to account for BGP packets with different TTL or IP options—those unmatched packets cascade to the class-default, which is policed at only 10 pps. This low rate cannot sustain the BGP session’s keepalives and updates, leading to flapping. On the CCNP ENARSI 300-410 exam, this scenario tests your understanding that CoPP class-default acts as a catch-all and that ACLs must be exhaustive, especially in MPLS L3VPN environments where control-plane traffic can include labeled BGP packets. A common trap is assuming a correct ACL syntax guarantees matching; the real issue is completeness. Memory tip: “Default drops what the ACL forgot”—always verify your match criteria cover both directions and all packet variations.
300-410 MPLS L3VPN Practice Question
This 300-410 practice question tests your understanding of mpls l3vpn. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 PE router in an MPLS L3VPN to protect the control plane. The engineer creates a policy-map that matches BGP packets and applies a police rate of 100 pps. The engineer also configures a class-default with a police rate of 10 pps. After applying the policy to the control-plane, the engineer notices that BGP sessions are flapping and the router is dropping BGP packets. The engineer verifies that the ACL for BGP is correct. 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 policy is dropping BGP packets that are not matched by the BGP class due to an incomplete ACL.
A common edge case with CoPP is that the class-default policy can inadvertently match control plane traffic that is not explicitly matched in other classes. If the class-default has a very low police rate (e.g., 10 pps), it can drop packets that are not matched by the BGP class, such as routing protocol packets that are not BGP (e.g., OSPF, EIGRP) or management traffic. However, the specific issue here is that the BGP class is matching BGP packets, but if the ACL is not correctly matching all BGP packets (e.g., using TCP port 179 but not matching both directions), some BGP packets may fall into class-default and be dropped. The most likely explanation is that the class-default is dropping packets that are not explicitly matched, including some BGP packets that are not caught by the ACL.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 policy is dropping BGP packets that are not matched by the BGP class due to an incomplete ACL.
Why this is correct
Correct. If the ACL does not match all BGP packets (e.g., only matching TCP port 179 from one direction), some BGP packets fall into class-default and are dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The police rate for BGP is too low, causing BGP packets to be dropped even when matched.
Why it's wrong here
Incorrect. The BGP police rate is 100 pps, which is typically sufficient for BGP keepalives and updates. The flapping is due to dropped packets from class-default.
- ✗
The CoPP policy is applied to the wrong control-plane (e.g., 'control-plane host' instead of 'control-plane transit').
Why it's wrong here
Incorrect. Applying to the wrong control-plane would affect different traffic, but the symptom would still be BGP flapping if the policy were applied to host.
- ✗
The 'service-policy' command is missing the 'input' keyword, causing the policy to be applied only to output traffic.
Why it's wrong here
Incorrect. The 'input' keyword is required for CoPP, but if it were missing, the policy would not be applied at all, and BGP would not flap.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Trap categories for this question
Keyword trap
Incorrect. The 'input' keyword is required for CoPP, but if it were missing, the policy would not be applied at all, and BGP would not flap.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
MPLS L3VPN — This question tests MPLS L3VPN — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The class-default policy is dropping BGP packets that are not matched by the BGP class due to an incomplete ACL. — A common edge case with CoPP is that the class-default policy can inadvertently match control plane traffic that is not explicitly matched in other classes. If the class-default has a very low police rate (e.g., 10 pps), it can drop packets that are not matched by the BGP class, such as routing protocol packets that are not BGP (e.g., OSPF, EIGRP) or management traffic. However, the specific issue here is that the BGP class is matching BGP packets, but if the ACL is not correctly matching all BGP packets (e.g., using TCP port 179 but not matching both directions), some BGP packets may fall into class-default and be dropped. The most likely explanation is that the class-default is dropping packets that are not explicitly matched, including some BGP packets that are not caught by the ACL.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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