The answer is that CoPP is dropping the locally generated ERSPAN packets because the router treats these mirrored packets as control plane traffic. When R1 generates ERSPAN-encapsulated copies of monitored traffic, those packets are sourced from the router itself and must be processed by the control plane for routing and encapsulation before being sent out. If the CoPP policy applies to all incoming control plane traffic and does not include an exception for locally originated ERSPAN packets, the rate-limiter will police and drop them before they ever leave the router. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that ERSPAN packets are not transit traffic—they are locally generated, so CoPP can inadvertently block them even when the destination is reachable. A common trap is assuming CoPP only affects traffic destined to the router’s IP, but ERSPAN packets hit the control plane during generation. Memory tip: “Locally generated, control plane regulated—if CoPP is applied, ERSPAN gets policed.”
300-410 SPAN, RSPAN, and ERSPAN Practice Question
This 300-410 practice question tests your understanding of span, rspan, and erspan. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Router R1 is configured as an ERSPAN source to monitor traffic on interface Serial0/0/0. The destination is a collector at 172.16.1.100. R1's configuration: monitor session 1 type erspan-source source interface Serial0/0/0 both destination erspan-id 1 ip address 172.16.1.100 origin ip address 10.0.0.1. The collector is reachable via a default route. The network also uses CoPP (Control Plane Policing) with a policy that polices all incoming traffic to the control plane. The collector receives no packets. What is the most likely issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
CoPP is policing the locally generated ERSPAN packets, causing them to be dropped.
ERSPAN encapsulated packets are sent to the destination IP using the routing table. When the router sends these packets, they are generated by the router itself (locally originated). CoPP typically applies to traffic destined to the control plane, not to transit traffic. However, ERSPAN packets are locally generated and may be subject to CoPP if the policy is applied to the control plane input. Specifically, if the CoPP policy classifies all IP traffic to the control plane, the ERSPAN packets might be policed and dropped. But more subtly, the ERSPAN process may use the control plane to generate the packets, and if the CoPP rate-limits the control plane, the packets might be dropped. The correct root cause is that CoPP is rate-limiting the ERSPAN traffic because the router treats the mirrored packets as control plane traffic.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 ERSPAN destination IP is not in the routing table, so packets are dropped.
CoPP is policing the locally generated ERSPAN packets, causing them to be dropped.
Why this is correct
ERSPAN packets are locally generated and may be subject to CoPP if the policy applies to control plane input.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The ERSPAN session is not configured with 'no shutdown'.
Why it's wrong here
The configuration shows no 'shutdown' command, so it is enabled by default.
✗
The source interface Serial0/0/0 is down, so no traffic is mirrored.
Why it's wrong here
The symptom is that the collector receives no packets, but the interface may be up; the scenario does not indicate it is down.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Trap categories for this question
Command / output trap
The configuration shows no 'shutdown' command, so it is enabled by default.
Scenario analysis trap
The symptom is that the collector receives no packets, but the interface may be up; the scenario does not indicate it is down.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
SPAN, RSPAN, and ERSPAN — This question tests SPAN, RSPAN, and ERSPAN — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: CoPP is policing the locally generated ERSPAN packets, causing them to be dropped. — ERSPAN encapsulated packets are sent to the destination IP using the routing table. When the router sends these packets, they are generated by the router itself (locally originated). CoPP typically applies to traffic destined to the control plane, not to transit traffic. However, ERSPAN packets are locally generated and may be subject to CoPP if the policy is applied to the control plane input. Specifically, if the CoPP policy classifies all IP traffic to the control plane, the ERSPAN packets might be policed and dropped. But more subtly, the ERSPAN process may use the control plane to generate the packets, and if the CoPP rate-limits the control plane, the packets might be dropped. The correct root cause is that CoPP is rate-limiting the ERSPAN traffic because the router treats the mirrored packets as control plane traffic.
What should I do if I get this 300-410 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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