350-501 MPLS and Segment Routing Practice Question
A service provider is deploying MPLS L2VPN using Virtual Private Wire Service (VPWS). After configuration, the two CEs cannot ping each other. The PE routers show the xconnect interface is up, but no MAC addresses are learned. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between local interface status (attachment circuit up) and pseudowire status (VC ID match required), leading candidates to mistakenly focus on MTU or encapsulation issues when the real problem is a control plane mismatch.
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 VC ID mismatch between the two PEs
In MPLS L2VPN VPWS, the VC ID (Virtual Circuit ID) must match on both PE routers to establish the pseudowire between them. A mismatch prevents the pseudowire from coming up, so even though the local xconnect interface shows as up (because the attachment circuit is operational), no MAC learning can occur because the pseudowire control plane (e.g., LDP or BGP signaled) fails to bind the two endpoints. This directly explains why the CEs cannot ping and no MAC addresses are learned.
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 MTU on the core links is less than 1500
Why it's wrong here
MTU mismatch would cause fragmentation, but pseudowire would still be up.
- ✗
The control word is enabled on one PE but not the other
Why it's wrong here
Control word mismatch can cause issues, but typically the pseudowire would be up with errors.
- ✓
The VC ID mismatch between the two PEs
Why this is correct
VC ID must match for the pseudowire to be established.
- ✗
The encapsulation type on the AC is different on the two PEs
Why it's wrong here
Encapsulation must match, but both are probably Ethernet, and the question implies it's up.
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
Go deeper
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JA
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.