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.”
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 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The monitoring server does not have a route to the tunnel source IP 10.0.0.1.
ERSPAN uses GRE encapsulation, and the tunnel destination must be the IP address of the monitoring server that can decapsulate GRE. However, the tunnel source IP must be reachable from the server; if the server does not have a route back to the tunnel source, the encapsulated packets may be dropped.
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 monitoring server does not have a route to the tunnel source IP 10.0.0.1.
Why this is correct
Correct because the server needs to be able to respond to or process GRE packets; if it cannot reach the tunnel source, the packets may be discarded.
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 configured with the wrong direction.
Why it's wrong here
Incorrect because ERSPAN captures traffic bidirectionally by default; direction is not the issue.
✗
The router does not support ERSPAN.
Why it's wrong here
Incorrect because the engineer configured it; the issue is likely a routing problem.
✗
The monitoring server is not listening on the correct TCP port.
Why it's wrong here
Incorrect because ERSPAN uses GRE, not TCP; the server must support GRE decapsulation.
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.
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: The monitoring server does not have a route to the tunnel source IP 10.0.0.1. — ERSPAN uses GRE encapsulation, and the tunnel destination must be the IP address of the monitoring server that can decapsulate GRE. However, the tunnel source IP must be reachable from the server; if the server does not have a route back to the tunnel source, the encapsulated packets may be dropped.
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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