- A
The ERSPAN session is missing a tunnel interface configuration.
Correct because ERSPAN encapsulates monitored traffic in GRE tunnels; a tunnel interface must be configured and referenced in the monitor session.
- B
The monitoring server is not listening on the correct UDP port.
Why wrong: Incorrect because ERSPAN uses GRE encapsulation, not UDP; the server must be able to decapsulate GRE packets.
- C
The source interface is not in the same subnet as the destination IP.
Why wrong: Incorrect because IP connectivity is verified; the issue is not about subnet matching.
- D
The ERSPAN session is configured with the wrong source IP address.
Why wrong: Incorrect because the source IP is used for the tunnel; while possible, the primary missing element is the tunnel interface itself.
Quick Answer
The answer is that the ERSPAN session is missing a tunnel interface configuration. ERSPAN, or Encapsulated Remote Switched Port Analyzer, works by taking a copy of monitored traffic and encapsulating it within GRE packets for transport across a Layer 3 network to a remote destination. Without a dedicated tunnel interface configured on the source router, the ERSPAN session has no logical encapsulation point to build those GRE headers, so even though IP connectivity exists between Router A and the monitoring server at 10.1.1.100, the captured packets never get properly encapsulated and thus never leave the router. On the Cisco CCNP ENARSI 300-410 exam, this is a classic troubleshooting trap: engineers often verify reachability and source session configuration but forget that ERSPAN requires an explicit tunnel interface with a matching source and destination under the monitor session parameters. A helpful memory tip is "No tunnel, no bubble"—without the tunnel interface, the ERSPAN packets have no envelope to travel across the network.
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.
A network engineer is troubleshooting an ERSPAN configuration where traffic from a source router is being sent to a remote monitoring server. The engineer configures an ERSPAN source session on Router A to capture traffic on GigabitEthernet0/0 and send it to the IP address 10.1.1.100. The monitoring server does not receive any packets. The engineer verifies that IP connectivity exists between Router A and the server. 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 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 missing a tunnel interface configuration.
ERSPAN requires a tunnel interface to encapsulate the monitored traffic. Without configuring a tunnel interface, the ERSPAN session cannot encapsulate the packets for transport to the remote destination.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 session is missing a tunnel interface configuration.
Why this is correct
Correct because ERSPAN encapsulates monitored traffic in GRE tunnels; a tunnel interface must be configured and referenced in the monitor session.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The monitoring server is not listening on the correct UDP port.
Why it's wrong here
Incorrect because ERSPAN uses GRE encapsulation, not UDP; the server must be able to decapsulate GRE packets.
- ✗
The source interface is not in the same subnet as the destination IP.
Why it's wrong here
Incorrect because IP connectivity is verified; the issue is not about subnet matching.
- ✗
The ERSPAN session is configured with the wrong source IP address.
Why it's wrong here
Incorrect because the source IP is used for the tunnel; while possible, the primary missing element is the tunnel interface itself.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
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SPAN, RSPAN, and ERSPAN — study guide chapter
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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 — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The ERSPAN session is missing a tunnel interface configuration. — ERSPAN requires a tunnel interface to encapsulate the monitored traffic. Without configuring a tunnel interface, the ERSPAN session cannot encapsulate the packets for transport to the remote destination.
What should I do if I get this 300-410 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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Last reviewed: Jun 19, 2026
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