Quick Answer
The answer is split horizon, the mechanism in VPLS that prevents loops by ensuring a PE never forwards a frame received from one pseudowire out another pseudowire within the same VFI. This directly stops broadcast or unknown unicast frames from being reflected back to the originating PE, which would otherwise cause MAC flapping and broadcast storms. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this concept tests your understanding of Layer 2 VPN loop prevention in the MPLS core, often appearing as a distractor where STP or RSTP is incorrectly chosen—remember those protocols operate on the CE side, not inside the VPLS core. A common trap is confusing MAC aging with loop prevention, but aging only handles stale entries, not loop topology. Memory tip: think of split horizon as “one pseudowire in, never out another pseudowire” within the same VFI—like a one-way street for traffic between PEs.
This 350-501 practice question tests your understanding of services. 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.
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?
Split horizon
Split horizon prevents forwarding out of incoming pseudowire.
Spanning Tree Protocol (STP)
Why wrong: STP runs on CE devices, not PEs.
Rapid Spanning Tree Protocol (RSTP)
Why wrong: Same as above.
MAC address aging
Why wrong: MAC aging removes stale entries but does not prevent loops.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Split horizon
Option D is correct. Split horizon in VPLS means a PE will not forward a frame received from one pseudowire out another pseudowire within the same VFI, preventing loops. Option A (STP) is used at the CE side but not inside the VPLS core. Option B (RSTP) same. Option C (MAC aging) is for learning, not loop prevention.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
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.
Related concept
CIDR notation defines the prefix length.
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.
Common exam traps
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
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
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 350-501 subnetting questions on CIDR, address ranges, and subnet selection.
Services — study guide chapter
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Services practice questions
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Cisco SPCOR / CCNP Service Provider Core 350-501 study guide
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FAQ
Services — This question tests Services — CIDR notation defines the prefix length..
The correct answer is: Split horizon — Option D is correct. Split horizon in VPLS means a PE will not forward a frame received from one pseudowire out another pseudowire within the same VFI, preventing loops. Option A (STP) is used at the CE side but not inside the VPLS core. Option B (RSTP) same. Option C (MAC aging) is for learning, not loop prevention.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 350-501 subnetting questions on CIDR, address ranges, and subnet selection.
CIDR notation defines the prefix length.
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Last reviewed: Jun 24, 2026
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