hardMultiple Choice
350-401 Control Plane Policing (CoPP) Practice Question
A network engineer is configuring CoPP on a Cisco router to protect the control plane from excessive traffic. The engineer creates a class-map that matches traffic with a specific ACL that permits TCP port 22 (SSH) from a management subnet (192.168.1.0/24) and denies all other traffic. The CoPP policy applies a police rate of 1 Mbps to this class. After applying the policy, the engineer notices that SSH sessions from the management subnet are being dropped intermittently. The engineer checks the CoPP statistics and sees that the traffic rate is 500 kbps. What is the most likely cause?
⚠ Common exam trap
A common pitfall is assuming that an average traffic rate below the police rate means there should be no drops. CoPP policing is based on bursts as well as average rate; intermittent drops often occur when bursts exceed the configured rate, even if the average rate is below the police rate.
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 police rate is too low, and the traffic is being dropped due to exceeding the rate.
The most likely cause is that traffic is exceeding the police rate in bursts, even though the average rate is 500 kbps and the configured police rate is 1 Mbps. Policing uses a token bucket, and TCP SSH traffic from the management subnet can burst above the configured rate. When bursts exceed the police rate, the exceed-action (by default, drop) drops the excess packets, causing intermittent SSH drops. If the conform-action were set to drop, all conforming traffic would be dropped consistently, not intermittently. The default conform-action is transmit, and there is no indication it was changed.
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 has a conform-action of drop, which drops all traffic matching the class.
Why it's wrong here
A conform-action of drop would instruct the policer to drop packets that are within the allowed rate, meaning every SSH packet that conforms to the token bucket would be discarded immediately. That would produce a total and consistent loss of SSH connectivity from the management subnet, not the intermittent drops described. In a typical CoPP policy, the conform-action is transmit so that only excess traffic is subject to drop, and the default conform-action for a police command is transmit unless explicitly changed.
- ✓
The police rate is too low, and the traffic is being dropped due to exceeding the rate.
Why this is correct
The police rate alone can cause drops even when the average bit rate is below the configured police rate because CoPP uses a token bucket that only allows short bursts up to the burst size. Once the bucket is empty, every packet that exceeds the sustained rate or even a momentary burst is marked as exceeding and is dropped by the exceed-action. Intermittent drops are the classic symptom of a policer with a low rate or small burst parameters, not a config error.
- ✗
The ACL is denying SSH traffic from the management subnet.
Why it's wrong here
The class map for SSH traffic references an ACL that contains an explicit permit statement for TCP port 22 sourced from 192.168.1.0/24, which is the management subnet. Since the permit matches the management station's traffic, the ACL cannot be the cause of the drops; instead, it is the prerequisite for classifying the traffic into the SSH class. Therefore, denying SSH would require a deny statement or a missing permit, which is not the case here.
- ✗
The CoPP policy is applied to the wrong interface, so it is not affecting SSH traffic.
Why it's wrong here
CoPP policies are attached to the control plane interface using the 'control-plane' global configuration, not to an individual Layer 3 physical interface. Because the policy is applied globally to the control plane, the concept of being applied to the wrong interface does not apply; all traffic destined to the router's control plane is evaluated by the policy. If the policy were mistakenly attached only to a data-plane interface, it would not affect control-plane traffic at all, but that scenario is not present in this environment.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.