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300-410 Practice Question: Configures CoPP on a router to limit ICMP traffic…
A network engineer configures CoPP on a router to limit ICMP traffic to 5000 bps. After the policy is applied, the engineer notices that the router is not responding to ping requests from a remote network. However, the router can ping other devices successfully. The engineer checks the CoPP statistics and sees that the ICMP class has dropped packets. What is the most likely root cause?
⚠ Common exam trap
Many candidates confuse the direction of CoPP policing, assuming it applies to both inbound and outbound control plane traffic, when in fact only inbound traffic to the control plane is policed by default.
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 is dropping incoming ICMP echo requests because the police rate is too low.
The CoPP policy is applied to control plane traffic, which includes ICMP echo requests destined to the router itself. When the police rate is set to 5000 bps, incoming ICMP echo requests are rate-limited and dropped if they exceed this threshold, causing the router to stop responding to pings. The fact that the router can still ping other devices confirms that the data plane is unaffected, isolating the issue to control plane policing of inbound ICMP.
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 is dropping incoming ICMP echo requests because the police rate is too low.
Why this is correct
Incoming ICMP packets are policed by CoPP, and if the rate is exceeded, they are dropped, preventing the router from responding.
- ✗
The CoPP policy is dropping outgoing ICMP echo replies because the police rate applies to both directions.
Why it's wrong here
CoPP applies to incoming traffic to the control plane, not outgoing traffic.
- ✗
The router's interface ACL is blocking incoming ICMP traffic.
Why it's wrong here
The scenario does not mention an ACL; CoPP is the likely cause.
- ✗
The router's ICMP rate-limit feature is enabled globally.
Why it's wrong here
ICMP rate-limit is a separate feature; the scenario focuses on CoPP.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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