easyMultiple Choice
300-410 Practice Question: An engineer applies a CoPP policy to a router to…
An engineer applies a CoPP policy to a router to protect the control plane from a DDoS attack. The policy includes a class-map matching UDP traffic to port 123 (NTP) and polices it to 1000 bps. After the policy is applied, the engineer notices that the router's clock is not synchronizing with its NTP server. The NTP server is reachable via ping. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the misconception that a CoPP policy will only drop malicious traffic, but the trap here is that an overly restrictive police rate can inadvertently drop legitimate control-plane traffic like NTP, even when the class-map and port numbers are correctly configured.
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 NTP packets because the police rate is too low.
The CoPP policy is policing NTP traffic (UDP port 123) to only 1000 bps. NTP synchronization requires a steady exchange of packets, and a 1000 bps rate is extremely low—likely insufficient to allow the NTP packets through, causing them to be dropped. Since the NTP server is reachable via ping (ICMP is not affected by this CoPP policy), the issue is clearly that the police rate is too restrictive for NTP traffic.
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 NTP packets because the police rate is too low.
Why this is correct
Policing NTP to 1000 bps is far below what synchronisation requires; NTP exchanges several packets per poll, so the policer drops excess, preventing clock sync even though ICMP ping succeeds because it is not matched by the NTP class.
- ✗
The NTP server is not responding because of the DDoS attack.
Why it's wrong here
Ping success proves the server is reachable and responding, so a DDoS-induced outage contradicts the stem; the policer itself is dropping NTP. It is tempting because CoPP exists to mitigate DDoS, and would be correct if the server were unreachable or its replies were being dropped upstream.
- ✗
The CoPP class-map is not matching NTP packets because it uses the wrong port number.
Why it's wrong here
Port 123 is the correct NTP port, so the class-map matches the traffic; the failure stems from the 1000 bps policer dropping synchronisation packets, not from a port mismatch. It is tempting because a wrong port would indeed prevent matching, and would be the answer if the class-map referenced 133 or 323 instead.
- ✗
The router's NTP configuration has a wrong server IP address.
Why it's wrong here
A wrong server address would prevent synchronisation, but ping to the NTP server succeeds, confirming the configured address is reachable; the policer is the actual cause. It is tempting because misconfigured server addresses are a common NTP fault, and would be correct if the router could not reach the server at all.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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