Question 804 of 2,152

Control Plane Policing (CoPP) →hardMultiple ChoiceObjective-mapped

CoPP DMVPN Troubleshooting — NHRP Redirect Drops | Cisco CCNP ENARSI 300-410 Explained

This 300-410 practice question tests your understanding of control plane policing (copp). 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.

A network engineer configures CoPP on a router that is a DMVPN hub. The policy includes a class-map to match NHRP traffic and police it. After deployment, spoke-to-spoke tunnels fail to establish, although spoke-to-hub tunnels work. 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
The CoPP policy drops IPsec packets, which are used for spoke-to-spoke encryption.
Why wrong: Spoke-to-spoke tunnels use mGRE, not IPsec directly; NHRP is the control protocol.
B
The CoPP policy polices NHRP traffic, causing NHRP redirect packets from the hub to be dropped, so spokes cannot learn each other's addresses.
NHRP redirects are essential for spoke-to-spoke communication; policing them breaks the dynamic tunnel setup.
C
The CoPP policy is applied to the tunnel interface, not the control plane.
Why wrong: CoPP is applied to the control plane, not interfaces.
D
The CoPP policy uses the default class class-default, which blocks NHRP.
Why wrong: The class-default permits traffic by default.

Full breakdown with real-world context →

Quick Answer

The answer is that the CoPP policy polices NHRP traffic, causing NHRP redirect packets from the hub to be dropped, so spokes cannot learn each other's addresses. In DMVPN Phase 2, spoke-to-spoke tunnel establishment relies on the hub sending NHRP redirect messages to inform a source spoke of a destination spoke’s real IP address; if CoPP aggressively polices NHRP packets, these critical redirects are dropped, breaking the spoke-to-spoke path while leaving spoke-to-hub traffic unaffected. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how control-plane protection interacts with DMVPN dynamic routing—a common trap is assuming the issue lies with IPsec or mGRE misconfiguration rather than CoPP. Remember the key chain: CoPP drops NHRP redirects, so spokes stay blind to each other. A useful memory tip is “Redirects need respect—police them and spokes disconnect.”

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 CoPP policy polices NHRP traffic, causing NHRP redirect packets from the hub to be dropped, so spokes cannot learn each other's addresses.

In a DMVPN hub-and-spoke topology, spoke-to-spoke tunnels rely on NHRP redirect messages from the hub to learn each other's public addresses. If CoPP polices NHRP traffic too aggressively, the hub may drop NHRP redirect packets before they reach the spokes. Without these redirects, spokes cannot initiate direct tunnels, even though spoke-to-hub communication (which uses the hub as a relay) remains functional.

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.

✗
The CoPP policy drops IPsec packets, which are used for spoke-to-spoke encryption.
Why it's wrong here
Spoke-to-spoke tunnels use mGRE, not IPsec directly; NHRP is the control protocol.
✓
The CoPP policy polices NHRP traffic, causing NHRP redirect packets from the hub to be dropped, so spokes cannot learn each other's addresses.
Why this is correct
NHRP redirects are essential for spoke-to-spoke communication; policing them breaks the dynamic tunnel setup.
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 CoPP policy is applied to the tunnel interface, not the control plane.
Why it's wrong here
CoPP is applied to the control plane, not interfaces.
✗
The CoPP policy uses the default class class-default, which blocks NHRP.
Why it's wrong here
The class-default permits traffic by default.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the distinction between control-plane and data-plane traffic in DMVPN; the trap here is that candidates assume spoke-to-spoke failures are caused by IPsec or tunnel interface issues, when the root cause is actually the policing of NHRP control-plane messages that enable dynamic tunnel establishment.

Detailed technical explanation

How to think about this question

NHRP redirect messages are generated by the hub when it forwards a packet from one spoke to another, informing the source spoke of the destination spoke's NBMA address. CoPP policies that police NHRP packets (e.g., using 'match protocol nhrp' or a class-map matching UDP port 434) can inadvertently drop these redirects if the policer rate is too low. In real-world deployments, engineers often forget to exempt NHRP redirects or set an adequate policer rate, leading to partial DMVPN failure where spoke-to-hub works but spoke-to-spoke does not.

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: The CoPP policy polices NHRP traffic, causing NHRP redirect packets from the hub to be dropped, so spokes cannot learn each other's addresses. — In a DMVPN hub-and-spoke topology, spoke-to-spoke tunnels rely on NHRP redirect messages from the hub to learn each other's public addresses. If CoPP polices NHRP traffic too aggressively, the hub may drop NHRP redirect packets before they reach the spokes. Without these redirects, spokes cannot initiate direct tunnels, even though spoke-to-hub communication (which uses the hub as a relay) remains functional.

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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