350-601 Network Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
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.
Why it's wrong here
If LACP were misconfigured, the port-channel would not be up.
- ✗
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.
Visual reference
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 |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-601 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 350-601 exam.