STP Blocking Ports: Troubleshooting After Switch Replacement
A network technician is connecting two network switches in a small office to extend the wired network. The technician uses two cables between the switches for redundancy, but the network becomes unstable and devices experience intermittent connectivity. Which of the following protocols should the technician implement to prevent network loops while allowing the redundant links?
⚠ Common exam trap
CompTIA often tests the misconception that VLANs or link aggregation (not listed) can replace STP, but VLANs do not eliminate loops—they only confine broadcast traffic, so STP is still required per VLAN (or PVST+).
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 (STP)
Spanning Tree Protocol (STP) prevents network loops by placing redundant links into a blocking state while maintaining a single active path between switches. When a primary link fails, STP automatically recalculates the topology and transitions a backup link to forwarding, ensuring redundancy without broadcast storms or MAC table instability.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Spanning Tree Protocol (STP)
Why this is correct
Spanning Tree Protocol is correct because it actively prevents Layer 2 loops by computing a loop-free logical tree over physically redundant switch links. Switches exchange BPDUs to elect a root bridge, then place non-designated ports into a blocking state; these ports only transition to forwarding if the active path fails, which eliminates loops while preserving redundancy. Without STP, broadcast frames would circulate endlessly between the two switches, consuming bandwidth and destabilizing MAC tables.
- ✗
VLAN (Virtual LAN)
Why it's wrong here
VLANs are incorrect because they separate traffic into distinct broadcast domains at Layer 2, but they do not remove the physical cabling loop between the switches. A loop still exists in each VLAN's topology, so packets can ping-pong across the switches indefinitely; VLANs may segment which frames loop, but they cannot block or disable the redundant port. STP still must run within each VLAN (or on a per-VLAN basis) to prevent those physical loops.
- ✗
DHCP (Dynamic Host Configuration Protocol)
Why it's wrong here
DHCP is incorrect because it is a client-server protocol that dynamically assigns IP addresses, subnet masks, gateways, and DNS settings to end hosts. Loop prevention is a Layer 2 switching function concerned with frame forwarding paths, not network-layer address management. In fact, a switch loop can cause a broadcast storm that severely degrades or disables DHCP responses, so DHCP relies on loop prevention rather than providing it.
- ✗
NAT (Network Address Translation)
Why it's wrong here
NAT is incorrect because it modifies IP address information in packet headers as traffic crosses a router or firewall, typically to translate between private and public addresses. It operates at Layer 3 (and sometimes Layer 4), entirely above the Ethernet switching layer where loops occur. Since NAT does not influence bridge forwarding decisions or port states, it has no mechanism to break the loop between two directly connected switches.
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1101 practice question is part of Courseiva's free CompTIA 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 220-1101 exam.