STP Prevents Layer 2 Loops | Generated by AI
Question: Which protocol prevents Layer 2 switching loops, and please give a comprehensive explanation of each option (VTP, DTP, STP, LACP)?
Answer:
The correct answer is C) STP (Spanning Tree Protocol).
Comprehensive Explanation of Each Option
- A) VTP (VLAN Trunking Protocol)
VTP is a Cisco-proprietary Layer 2 messaging protocol used to manage the addition, deletion, and renaming of VLANs across multiple switches in the same VTP domain. It synchronizes VLAN information so that administrators do not have to configure the same VLANs manually on every switch.
Key points:- Operates in three modes: Server, Client, Transparent
- Reduces administrative overhead for VLAN management
- Does NOT prevent switching loops — in fact, misconfigured VTP can accidentally remove VLANs or cause connectivity issues, but it has no loop-prevention capability.
- B) DTP (Dynamic Trunking Protocol)
DTP is another Cisco-proprietary protocol that negotiates trunking (ISLs or 802.1Q) between two switches automatically. It determines whether a link should become a trunk port or an access port and which trunking encapsulation to use.
Key points:- Modes include Dynamic Auto, Dynamic Desirable, Trunk, Access, No-negotiate
- Simplifies trunk configuration
- Does NOT prevent loops — if DTP mistakenly forms a trunk where it shouldn’t, it can actually help create a Layer 2 loop by allowing unintended VLAN traffic to flow.
- C) STP (Spanning Tree Protocol) – IEEE 802.1D (and its successors)
STP is the industry-standard protocol specifically designed to prevent Layer 2 switching loops in Ethernet networks with redundant paths.
How it works:- Elects a root bridge (switch with the lowest Bridge ID)
- Places redundant ports into blocking state so only one active path exists between any two points
- Listens for topology changes and reconverges if a link fails
- Modern variants: RSTP (802.1w), MSTP (802.1s), PVST+, RPVST+ (Cisco enhancements) provide faster convergence
Key role: Without STP (or a successor), broadcast storms, MAC table instability, and duplicate frame copies can crash the entire Layer 2 domain in seconds when redundant links exist.
- D) LACP (Link Aggregation Control Protocol) – IEEE 802.3ad / 802.1AX
LACP is a protocol that manages Link Aggregation Groups (EtherChannel/port-channel). It dynamically negotiates and maintains bundled links between two devices to increase bandwidth and provide redundancy.
Key points:- Bundles multiple physical links into one logical link
- Provides load balancing and automatic failover
- Does NOT prevent loops by itself — in fact, when configuring EtherChannel, STP treats the entire bundle as a single logical link. If EtherChannel is misconfigured (different LACP settings on each side), it can cause loops or blackholing.
Summary Table
| Option | Full Name | Primary Purpose | Prevents Layer 2 Loops? |
|---|---|---|---|
| A | VTP | VLAN database synchronization | No |
| B | DTP | Automatic trunk negotiation | No |
| C | STP | Block redundant paths to stop loops | Yes |
| D | LACP | Link aggregation (EtherChannel) | No |
Conclusion: Only STP (and its modern variants) is specifically designed and responsible for preventing Layer 2 switching loops in Ethernet networks. The other protocols serve entirely different purposes.