The answer is a Layer 2 loop caused by Spanning Tree Protocol failing to block a redundant path. When the MAC address table on the Nexus 7000 in DC1 shows the same server MAC on both the dark fiber port-channel and a local access port, it confirms that frames are circulating between the two data centers, causing MAC flapping and intermittent drops. In a vPC environment with a stretched Layer 2 domain over a dark fiber link, STP must block one side of the loop; if misconfigured or if the vPC peer-link and the dark fiber port-channel create an unblocked triangle, traffic loops. On the Cisco DCCOR 350-601 exam, this scenario tests your understanding of vPC loop prevention and STP interaction—a common trap is assuming LACP alone prevents loops, but it only manages link aggregation, not topology loops. Remember: vPC does not replace STP; it relies on it to block redundant Layer 2 extensions. Memory tip: “Same MAC on two ports? STP is short.”
350-601 Network Practice Question
This 350-601 practice question tests your understanding of network. 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.
You are a network engineer at a financial institution. The company has two data centers: DC1 and DC2, connected via a dark fiber link. Each data center has a pair of Nexus 7000 switches in a vPC configuration. The dark fiber link connects to a port on each Nexus 7000 pair using a Layer 2 port-channel. The requirement is to extend VLAN 100 between the two data centers for a critical application that requires a stretched Layer 2 domain. The current configuration has the port-channel on both sides set to mode 'active' with LACP. VLAN 100 is allowed on the trunk. The application servers report intermittent connectivity issues, with some packets being dropped. Upon inspection, you notice that the MAC address table on the Nexus 7000 in DC1 shows the MAC address of the server in DC2 on the dark fiber port-channel interface, but also on a local access port connected to a different server in the same VLAN. What is the most likely cause of the intermittent connectivity?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Spanning Tree Protocol is not blocking one of the redundant paths, creating a loop.
The MAC address table showing the same MAC address on both the dark fiber port-channel and a local access port indicates a Layer 2 loop. In a vPC environment with a Layer 2 extension between data centers, Spanning Tree Protocol (STP) should block one of the redundant paths to prevent loops. If STP fails to block the appropriate port, frames loop, causing MAC address flapping and intermittent packet drops.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 dark fiber link is experiencing high latency, causing MAC address timeouts.
Why it's wrong here
High latency would not cause the same MAC to appear on two different interfaces.
✗
LACP is misconfigured on one side, causing the port-channel to operate as individual links.
There is an asymmetric routing issue between the data centers.
Why it's wrong here
Asymmetric routing is Layer 3; the issue is Layer 2.
✓
Spanning Tree Protocol is not blocking one of the redundant paths, creating a loop.
Why this is correct
A loop causes MAC flapping and intermittent connectivity.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often attribute intermittent connectivity to LACP or routing issues, but the key clue is the MAC address appearing on two different interfaces in the same VLAN, which is a definitive sign of a Layer 2 loop that STP should have prevented.
Detailed technical explanation
How to think about this question
In a vPC domain, STP runs on the vPC peer-link and orphan ports, but the vPC member ports are treated as a single logical interface. When extending VLANs between data centers over a Layer 2 port-channel, a loop can form if there is an alternate path (e.g., a backup link or misconfiguration) that STP does not block. The MAC address flapping occurs because the switch sees the same MAC address on two different interfaces within the same VLAN, causing the switch to continuously update its MAC table, leading to intermittent connectivity. The 'show spanning-tree vlan 100' command would reveal the blocking state of the redundant link.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network — This question tests Network — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Spanning Tree Protocol is not blocking one of the redundant paths, creating a loop. — The MAC address table showing the same MAC address on both the dark fiber port-channel and a local access port indicates a Layer 2 loop. In a vPC environment with a Layer 2 extension between data centers, Spanning Tree Protocol (STP) should block one of the redundant paths to prevent loops. If STP fails to block the appropriate port, frames loop, causing MAC address flapping and intermittent packet drops.
What should I do if I get this 350-601 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". 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?
Read the scenario before looking for a memorised answer.
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