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300-410 Practice Question: Configures Control Plane Policing (CoPP) on a…
A network engineer configures Control Plane Policing (CoPP) on a DMVPN hub router to protect the control plane. The policy includes a class-map matching NHRP traffic and a police rate of 1000 pps. Unexpectedly, after applying the policy, NHRP registrations from spokes fail intermittently, and debug shows packets being dropped by CoPP. Which is the most likely explanation?
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's class-default has a lower police rate or is set to drop, and NHRP traffic is not explicitly matched in a higher class, causing it to fall into class-default and be dropped.
CoPP rate-limits control plane traffic. If the police rate is set in packets per second (pps), but the actual NHRP registration traffic is bursty (e.g., multiple spokes registering simultaneously), the policer may drop packets. The corner case is that the default CoPP class-default may also match NHRP traffic if not explicitly classified, and the class-default may have a lower rate or be set to drop. Additionally, CoPP uses a token bucket; if the rate is too low or the burst size is insufficient, packets are dropped. The engineer should ensure that NHRP traffic is matched in a dedicated class with appropriate rate and burst.
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's class-default has a lower police rate or is set to drop, and NHRP traffic is not explicitly matched in a higher class, causing it to fall into class-default and be dropped.
Why this is correct
If NHRP traffic is not classified in a specific class, it matches class-default, which may have a restrictive policy, leading to drops.
- ✗
The police rate of 1000 pps is too high for the hub's CPU, causing the router to drop packets due to CPU overload.
Why it's wrong here
CoPP protects the CPU; a high rate would allow more traffic, not cause drops.
- ✗
The CoPP policy is applied to the wrong interface; it should be applied to the tunnel interface, not the physical interface.
Why it's wrong here
CoPP is applied globally to the control plane, not to interfaces; it filters traffic destined to the router itself.
- ✗
The NHRP packets are being classified as 'critical' traffic, and the CoPP policy has a lower priority for critical traffic.
Why it's wrong here
CoPP does not have priority levels; it uses class maps based on ACLs or DSCP.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
Control Plane Policing
Control Plane Policing is a Cisco security feature that protects a router or switch by rate-limiting the traffic that the device's processor must handle, preventing it from being overwhelmed.
Key term
DMVPN Phase 2
DMVPN Phase 2 is an advanced Cisco routing technology that allows spoke routers to communicate directly with one another without sending traffic through a central hub, using dynamic routing protocols and multipoint GRE tunnels.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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