350-501 Services Practice Question
An SP is troubleshooting an MPLS L2VPN VPLS network where MAC flapping is occurring between two PEs. Which mechanism in VPLS prevents loops and ensures that a broadcast frame from one PE is not reflected back to the originating PE?
⚠ Common exam trap
Cisco often tests the distinction between loop prevention mechanisms used in the customer-facing domain (STP/RSTP) versus those used in the service provider core (split horizon), and the trap here is assuming that STP or RSTP can be applied to the MPLS pseudowires, when in fact VPLS explicitly relies on split horizon to prevent loops within the full-mesh of PEs.
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
✓
Split horizon
In a VPLS environment, split horizon is the mechanism that prevents loops by ensuring that a broadcast, multicast, or unknown unicast frame received from a pseudowire (PW) is never forwarded back to any other PW on the same PE. This rule, defined in RFC 4761 and RFC 4762, stops the originating PE from receiving its own reflected frames, thus eliminating the MAC flapping that would occur if the frame were looped through the MPLS core.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Split horizon
Why this is correct
Split horizon prevents forwarding out of incoming pseudowire.
- ✗
Spanning Tree Protocol (STP)
Why it's wrong here
STP runs on CE devices, not PEs.
- ✗
Rapid Spanning Tree Protocol (RSTP)
Why it's wrong here
Same as above.
- ✗
MAC address aging
Why it's wrong here
MAC aging removes stale entries but does not prevent loops.
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.