The answer is a backup pseudowire label mismatch caused by the LDP session for the backup path being down. During an LDP to SR-MPLS migration, when SR is enabled alongside LDP, the primary pseudowire may correctly use SR labels, but the backup pseudowire can remain tied to LDP labels if its LDP session fails or is not updated. This mismatch breaks the backup path because the forwarding plane expects consistent label switching, and without a valid LDP session, the backup pseudowire cannot install the necessary LDP labels for failover traffic. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of MPLS coexistence strategies and the importance of synchronizing label distribution protocols across all paths. A common trap is assuming SR automatically overrides all LDP bindings, but backup pseudowires often require explicit signaling or gradual LDP decommissioning. Remember: primary SR, backup LDP equals a broken backup—always verify both sessions are alive.
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. 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 service provider is migrating from a legacy MPLS network using LDP to Segment Routing (SR-MPLS). They have enabled SR on all routers and are using IS-IS. The migration plan is to keep LDP running alongside SR during the transition. After enabling SR, some traffic is being forwarded using the SR path, but the network operator notices that some CEs in an L2VPN are experiencing packet loss during failover scenarios. Troubleshooting shows that the primary pseudowire is using SR labels, but the backup pseudowire is still using LDP labels. The backup path is not working correctly. 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.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The backup pseudowire is still using LDP labels and the LDP session for the backup path is down
During coexistence of LDP and SR, there might be a mismatch in label switching. The backup pseudowire might be using LDP labels that are not consistent with the SR forwarding plane. The most likely cause is that the backup path is not properly programmed to use SR labels, or the LDP session for the backup path is down. The correct action is to ensure that the pseudowire's backup signaling is using the same transport mechanisms as the primary, or to disable LDP gradually.
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 backup path uses a different IGP metric
Why it's wrong here
Metric difference would affect routing but not label allocation.
✓
The backup pseudowire is still using LDP labels and the LDP session for the backup path is down
Why this is correct
If LDP sessions are down for the backup path, the backup pseudowire cannot use SR labels, leading to packet loss.
Clue confirmation
The clue words "most likely", "primary" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The MTU on the backup path is smaller than on the primary
Why it's wrong here
MTU would cause fragmentation but not specifically for backup.
✗
The prefix-SID for the remote PE is missing in the SR-TE database
Why it's wrong here
SR-TE is not used here; pseudowires rely on IGP label distribution.
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.
What to study next
Got this wrong? Here's your next step.
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 350-501 NAT questions on configuration and troubleshooting.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The backup pseudowire is still using LDP labels and the LDP session for the backup path is down — During coexistence of LDP and SR, there might be a mismatch in label switching. The backup pseudowire might be using LDP labels that are not consistent with the SR forwarding plane. The most likely cause is that the backup path is not properly programmed to use SR labels, or the LDP session for the backup path is down. The correct action is to ensure that the pseudowire's backup signaling is using the same transport mechanisms as the primary, or to disable LDP gradually.
What should I do if I get this 350-501 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 350-501 NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "most likely", "primary". 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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