The answer is that ERSPAN on a tunnel interface captures the inner IP packet, not the encapsulated GRE or IPsec header. This occurs because the router mirrors traffic at the point where it enters or exits the tunnel—before encapsulation or after decapsulation—so the outer encapsulation is never seen by the monitor session. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of where ERSPAN sources its traffic in the forwarding path; a common trap is assuming the tunnel interface shows the full packet as it traverses the wire. To capture the encapsulated packet, you must source ERSPAN from the physical egress interface instead. Remember the memory tip: “Tunnel shows the inside, physical shows the ride.”
300-410 SPAN, RSPAN, and ERSPAN Practice Question
This 300-410 practice question tests your understanding of span, rspan, and erspan. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
An engineer configures ERSPAN on a Cisco router to monitor traffic from a tunnel interface. The mirrored traffic shows the tunnel's inner IP headers, but the outer encapsulation is missing. What is the most likely cause?
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
✓
ERSPAN on a tunnel interface captures the inner IP packet, not the encapsulated GRE/IPsec packet.
When monitoring a tunnel interface with ERSPAN, the router captures the traffic at the point where it enters the tunnel (before encapsulation) or exits (after decapsulation). To capture the encapsulated packets, the source must be the physical egress interface, not the tunnel interface. By default, ERSPAN on a tunnel interface shows the inner packet.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
ERSPAN on a tunnel interface captures the inner IP packet, not the encapsulated GRE/IPsec packet.
Why this is correct
The tunnel interface processes the inner packet; the outer encapsulation is added later on the physical interface.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The ERSPAN session is configured with 'erspan-type' that strips the outer header.
Why it's wrong here
There is no such configuration option.
✗
The collector is not configured to decode GRE headers.
Why it's wrong here
The issue is on the source, not the collector.
✗
The tunnel interface is in a VRF, causing the outer header to be removed.
Why it's wrong here
VRF does not affect encapsulation visibility.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
SPAN, RSPAN, and ERSPAN — This question tests SPAN, RSPAN, and ERSPAN — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: ERSPAN on a tunnel interface captures the inner IP packet, not the encapsulated GRE/IPsec packet. — When monitoring a tunnel interface with ERSPAN, the router captures the traffic at the point where it enters the tunnel (before encapsulation) or exits (after decapsulation). To capture the encapsulated packets, the source must be the physical egress interface, not the tunnel interface. By default, ERSPAN on a tunnel interface shows the inner packet.
What should I do if I get this 300-410 question wrong?
Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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?
Read the scenario before looking for a memorised answer.
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