- A
The monitoring server does not have a route to the tunnel source IP 10.0.0.1.
Why wrong: Incorrect. The monitoring server does not need a route to the tunnel source IP to receive ERSPAN packets; ERSPAN is a one-way traffic flow that only requires the server to decapsulate GRE. The ping success confirms reachability, so a missing return route is not the cause.
- B
The ERSPAN session is configured with the wrong direction.
Correct. If the ERSPAN session direction is misconfigured (e.g., monitoring only incoming traffic when the interesting traffic is outgoing), no packets will be captured and forwarded to the server. This is a common configuration error.
- C
The router does not support ERSPAN.
Why wrong: Incorrect. The router is a Cisco device capable of ERSPAN, and the configuration was applied without errors; the ping success further indicates that the router is functioning.
- D
The monitoring server is not listening on the correct TCP port.
Why wrong: Incorrect. ERSPAN uses GRE (IP protocol 47), not TCP. Port numbers are irrelevant to this technology.
ERSPAN: Monitoring Server Lacks Route to Tunnel Source
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. A key principle to apply: eRSPAN. 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 on interface GigabitEthernet0/0/0 and send it to a monitoring server at 192.168.1.100. The engineer configures the ERSPAN session with a tunnel source of 10.0.0.1 and a tunnel destination of 192.168.1.100. The monitoring server receives no packets. The engineer pings 192.168.1.100 from the router and succeeds. 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.
Quick Answer
The answer is that the monitoring server lacks a route to the tunnel source IP 10.0.0.1. This is because ERSPAN encapsulates mirrored traffic in a GRE tunnel, and while a successful ping from the router to the server confirms Layer 3 reachability for standard IP traffic, the ERSPAN-encapsulated packets require the server to have a return path to the tunnel source address. Without a route back to 10.0.0.1, the server cannot decapsulate the GRE packets or send acknowledgments, causing the monitoring session to fail silently. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of ERSPAN’s asymmetric routing requirement—a common trap is assuming a successful ping means the tunnel is fully functional. Remember the key distinction: ping tests unicast reachability to the destination, but ERSPAN demands bidirectional reachability to and from the tunnel source. A helpful memory tip is “ERSPAN needs a round-trip ticket for the source IP.”
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 ERSPAN session is configured with the wrong direction.
ERSPAN uses GRE encapsulation to send mirrored traffic to a monitoring server. The ping success indicates Layer 3 reachability to the server, so the issue is not routing. The most common misconfiguration is specifying the wrong traffic direction (e.g., rx instead of tx) in the ERSPAN session, which results in no packets being captured and sent. The server does not need a route to the tunnel source, as ERSPAN is one-way; it only requires GRE decapsulation capability.
Key principle: ERSPAN
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 monitoring server does not have a route to the tunnel source IP 10.0.0.1.
Why it's wrong here
Incorrect. The monitoring server does not need a route to the tunnel source IP to receive ERSPAN packets; ERSPAN is a one-way traffic flow that only requires the server to decapsulate GRE. The ping success confirms reachability, so a missing return route is not the cause.
- ✓
The ERSPAN session is configured with the wrong direction.
Why this is correct
Correct. If the ERSPAN session direction is misconfigured (e.g., monitoring only incoming traffic when the interesting traffic is outgoing), no packets will be captured and forwarded to the server. This is a common configuration error.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
ERSPAN
- ✗
The router does not support ERSPAN.
Why it's wrong here
Incorrect. The router is a Cisco device capable of ERSPAN, and the configuration was applied without errors; the ping success further indicates that the router is functioning.
- ✗
The monitoring server is not listening on the correct TCP port.
Why it's wrong here
Incorrect. ERSPAN uses GRE (IP protocol 47), not TCP. Port numbers are irrelevant to this technology.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Candidates often assume that the monitoring server must be able to route back to the tunnel source, but ERSPAN is unidirectional. The real issue is usually session direction or GRE decapsulation support on the server.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- ERSPAN
- GRE Decapsulation
- Session Direction
- One-way Tunnel
TExam Day Tips
- 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
ERSPAN
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. ERSPAN Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Review eRSPAN, then practise related 300-410 questions on the same topic to reinforce the concept.
- →
SPAN, RSPAN, and ERSPAN — study guide chapter
Learn the concepts, then practise the questions
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SPAN, RSPAN, and ERSPAN practice questions
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FAQ
Questions learners often ask
What does this 300-410 question test?
SPAN, RSPAN, and ERSPAN — This question tests SPAN, RSPAN, and ERSPAN — ERSPAN.
What is the correct answer to this question?
The correct answer is: The ERSPAN session is configured with the wrong direction. — ERSPAN uses GRE encapsulation to send mirrored traffic to a monitoring server. The ping success indicates Layer 3 reachability to the server, so the issue is not routing. The most common misconfiguration is specifying the wrong traffic direction (e.g., rx instead of tx) in the ERSPAN session, which results in no packets being captured and sent. The server does not need a route to the tunnel source, as ERSPAN is one-way; it only requires GRE decapsulation capability.
What should I do if I get this 300-410 question wrong?
Review eRSPAN, then practise related 300-410 questions on the same topic to reinforce the concept.
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?
ERSPAN
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Last reviewed: Jun 19, 2026
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