CoPP Default Class Causing OSPF Packet Drops
A Nexus switch experiences high CPU utilization due to excessive ICMP traffic. An engineer applies a CoPP policy that includes a class matching ICMP with a drop action. After applying, legitimate OSPF hello packets are also being dropped. What is the most likely cause?
Quick Answer
The answer is that OSPF packets match the default class which has a drop action. This occurs because the CoPP policy explicitly drops ICMP traffic via a class-map, but OSPF hello packets use IP protocol 89, which does not match that ICMP class. Any traffic not matching a user-defined class falls into the implicit default class; if that default class is configured with a drop action, all unmatched control-plane traffic—including OSPF—is discarded. On the Cisco DCCOR and CCNP Data Center Core 350-601 exam, this scenario tests your understanding of CoPP class-matching hierarchy and the critical role of the default class. A common trap is assuming that only the explicitly matched traffic (ICMP) is affected, forgetting that the default class governs everything else. To avoid this, remember the memory tip: “If the default drops, everything stops.” Always verify that your CoPP policy includes an explicit permit for essential routing protocols like OSPF (protocol 89) before applying a drop action to the default class.
⚠ Common exam trap
Cisco often tests the concept that the default class in CoPP is not automatically 'permit' and must be explicitly configured; the trap here is assuming that only the matched class (ICMP) is affected, while forgetting that unmatched traffic falls to the default class, which can have a drop action.
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
✓
OSPF packets match the default class which has a drop action
When a CoPP policy is applied, traffic that does not match any explicit class-map falls into the default class. If the default class has a drop action, all unmatched traffic—including OSPF hello packets (which use IP protocol 89)—will be dropped. The class-map matching ICMP (typically based on protocol or DSCP) does not match OSPF, so OSPF packets are handled by the default class, causing the observed behavior.
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 applied to the wrong interface
Why it's wrong here
CoPP is applied to the control plane, not an interface, so incorrect application is unlikely.
- ✗
The CoPP policy rate-limits all traffic including OSPF below its needed rate
Why it's wrong here
Rate-limiting would still allow some packets; complete dropping of OSPF indicates a drop action, not rate-limit exceed.
- ✓
OSPF packets match the default class which has a drop action
Why this is correct
If the default class action is drop, any traffic not explicitly matched (including OSPF) will be dropped. This is a common misconfiguration.
- ✗
The class-map matches multiple protocols including OSPF
Why it's wrong here
If the class-map explicitly matches ICMP via protocol or ACL, it should not match OSPF. However, a poorly designed ACL could cause this.
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Same concept, more angles
2 more ways this is tested on 350-601
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which control plane protection mechanism should be configured to limit the rate of BGP updates destined to the CPU of a Nexus 9000 switch to prevent CPU overload?
easy- A.VLAN Access Control Lists (VACLs)
- ✓ B.Control Plane Policing (CoPP)
- C.EtherChannel load balancing
- D.Switched Port Analyzer (SPAN)
Why B: Control Plane Policing (CoPP) is the correct mechanism because it directly filters and rate-limits control plane traffic, such as BGP updates, before it reaches the CPU of a Nexus 9000 switch. By applying a CoPP policy, you can protect the CPU from being overwhelmed by excessive BGP updates, ensuring stability and preventing denial-of-service conditions.
Variation 2. Which THREE of the following are valid methods to secure the control plane on a Cisco Nexus 9000 switch?
hard- ✓ A.Enable control plane policing (CoPP) to rate-limit control plane traffic.
- ✓ B.Configure management access lists to restrict SSH/SNMP access.
- ✓ C.Disable unused services such as HTTP/HTTPS server.
- D.Enable Bidirectional Forwarding Detection (BFD) on all interfaces.
- E.Implement routing protocol authentication (e.g., OSPF MD5).
Why A: Control plane policing (CoPP) is a valid method to secure the control plane on a Cisco Nexus 9000 switch. CoPP uses a policy map applied to the control plane to rate-limit or drop traffic destined to the supervisor module, protecting the CPU from excessive or malicious traffic. This is a direct control plane security mechanism defined in Cisco NX-OS.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-601 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 350-601 exam.