350-501 MPLS and Segment Routing Practice Question
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?
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 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.
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-501 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-501 exam.