Question 805 of 2,152

Control Plane Policing (CoPP) →hardMultiple ChoiceObjective-mapped

How CoPP Can Drop IKE Packets and Disrupt IPsec Tunnels

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 router has CoPP configured with a class-map that matches all traffic and polices it to 10000 pps. The router also has IPsec configured for a site-to-site VPN. After applying CoPP, the IPsec tunnel goes up, but traffic through the tunnel is intermittently dropped. 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 ESP packets, which are data plane traffic.
Why wrong: ESP packets are data plane and should not hit the control plane unless they are destined to the router.
B
CoPP drops IKE packets during rekey, causing the IPsec tunnel to fail temporarily.
IKE packets are control plane; if dropped, the tunnel may not rekey properly, causing traffic loss.
C
IPsec uses TCP, and CoPP only polices UDP.
Why wrong: IKE uses UDP, and ESP is IP protocol 50.
D
The CoPP policy is applied to the tunnel interface, not the control plane.
Why wrong: CoPP is applied to the control plane.

Full breakdown with real-world context →

Quick Answer

The correct answer is that CoPP drops IKE packets during rekey, causing the IPsec tunnel to fail temporarily. This occurs because Control Plane Policing (CoPP) applies a rate limit to all traffic destined for the router’s control plane, including IKE negotiation packets on UDP port 500. When the IPsec tunnel attempts to rekey, the router must exchange new IKE messages, but if CoPP polices these packets to a low rate like 10000 pps, the rekey process can be starved, causing the tunnel to drop and then rebuild, which intermittently disrupts user traffic. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how CoPP interacts with IPsec control plane traffic—a common trap is assuming CoPP only affects management protocols, when in fact it can throttle IKE and even encapsulated ESP/AH packets if they hit the control plane. Remember the memory tip: “CoPP can choke the key,” meaning if you police the control plane too aggressively, you’ll drop the IKE rekey and break the tunnel.

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

✓

CoPP drops IKE packets during rekey, causing the IPsec tunnel to fail temporarily.

Option B is correct because CoPP polices control plane traffic, and IKE (UDP 500/4500) is control plane traffic. During IPsec rekey, IKE packets are sent to the router's control plane for processing. If CoPP drops these IKE packets due to the 10000 pps policer, the rekey fails, causing the IPsec tunnel to temporarily go down until the next rekey attempt. The tunnel itself stays up initially, but traffic is intermittently dropped because the tunnel fails during rekey events.

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 ESP packets, which are data plane traffic.
Why it's wrong here
ESP packets are data plane and should not hit the control plane unless they are destined to the router.
✓
CoPP drops IKE packets during rekey, causing the IPsec tunnel to fail temporarily.
Why this is correct
IKE packets are control plane; if dropped, the tunnel may not rekey properly, causing traffic loss.
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.
✗
IPsec uses TCP, and CoPP only polices UDP.
Why it's wrong here
IKE uses UDP, and ESP is IP protocol 50.
✗
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.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the distinction between control plane and data plane traffic, and the trap here is that candidates mistakenly think CoPP affects all traffic (including ESP data plane) rather than only control plane traffic like IKE.

Detailed technical explanation

How to think about this question

CoPP uses a control-plane service policy that matches traffic destined to the router's control plane (e.g., routing protocols, management traffic, IKE). IKE packets (UDP 500/4500) are classified as control plane traffic because they are processed by the router's CPU to establish and maintain IPsec SAs. During rekey, the router must exchange IKE messages; if CoPP drops these due to policing, the SA expires and the tunnel drops until a new rekey succeeds. In real-world scenarios, a low pps policer can cause intermittent VPN outages that are hard to diagnose without monitoring CoPP drops via 'show policy-map control-plane'.

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.

Quick reference

VPN Protocol Comparison

Protocol	Port	Encryption	Authentication	Use Case
IKEv2 / IPsec	UDP 500 / 4500	AES-256	Certificates / PSK	Site-to-site & remote access
SSL / TLS VPN	TCP 443	TLS 1.3	Certificates / MFA	Clientless remote access
L2TP / IPsec	UDP 1701	AES (IPsec)	PSK / Certificates	Legacy remote access
WireGuard	UDP 51820	ChaCha20	Public keys	Modern high-performance VPN
PPTP	TCP 1723	MPPE (weak)	MS-CHAPv2	Legacy — avoid in production

PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.

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: CoPP drops IKE packets during rekey, causing the IPsec tunnel to fail temporarily. — Option B is correct because CoPP polices control plane traffic, and IKE (UDP 500/4500) is control plane traffic. During IPsec rekey, IKE packets are sent to the router's control plane for processing. If CoPP drops these IKE packets due to the 10000 pps policer, the rekey fails, causing the IPsec tunnel to temporarily go down until the next rekey attempt. The tunnel itself stays up initially, but traffic is intermittently dropped because the tunnel fails during rekey events.

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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