The answer is that the LDP session between the routers is not established, as the output explicitly shows "No remote LDP session" as the last error. This error occurs because MPLS L2VPN virtual circuits require a functioning Label Distribution Protocol session between the two provider edge routers to exchange label bindings for the pseudowire; without this session, the VC remains down regardless of interface status. On the Cisco CCNP ENARSI 300-410 exam, this question tests your ability to interpret "show mpls l2transport vc detail" output and differentiate between control-plane issues like LDP adjacency failure and data-plane issues like MTU mismatches. A common trap is to assume the problem is with the local interface or the underlying MPLS core, but the "last error" field pinpoints the root cause. Remember the memory tip: "No LDP, no VC" — if the remote LDP session is missing, the pseudowire cannot signal labels, keeping the virtual circuit down.
300-410 MPLS Operations Practice Question
This 300-410 practice question tests your understanding of mpls operations. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 runs the following command on Router R1:
R1# show mpls l2transport vc 100 detail
Local interface: Gi0/0/0 up, line protocol up
Destination: 10.0.0.2, VC ID: 100, VC status: down
Last error: No remote LDP session
Based on this output, what is the most likely cause of the VC being down?
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 LDP session between the routers is not established.
The output explicitly states 'No remote LDP session' as the last error. This indicates that the LDP session between the two routers (10.0.0.1 and 10.0.0.2) is not established, which is required for the pseudowire to come up.
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 local interface Gi0/0/0 is administratively down.
Why it's wrong here
The interface is up and line protocol up.
✗
The remote router 10.0.0.2 is not reachable via IP.
Why it's wrong here
The error is specifically about LDP session, not IP reachability.
✓
The LDP session between the routers is not established.
Why this is correct
The last error confirms 'No remote LDP session'.
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 VC ID 100 is mismatched on the remote router.
Why it's wrong here
VC ID mismatch would show a different error, like 'VC ID mismatch'.
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
VC ID mismatch would show a different error, like 'VC ID mismatch'.
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.
MPLS Operations — This question tests MPLS Operations — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The LDP session between the routers is not established. — The output explicitly states 'No remote LDP session' as the last error. This indicates that the LDP session between the two routers (10.0.0.1 and 10.0.0.2) is not established, which is required for the pseudowire to come up.
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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