How Spanning Tree Protocol Works to Prevent Network Loops and Ensure Security?

Spanning Tree Protocol is one of the crucial mechanisms that keeps your network reliable and secure. STP protects your network against potential issues. It provides a blueprint for efficiently navigating within the network. In this blog, we will dig into the fundamentals of Spanning Tree Protocol and explore how it enhances network security.

What is Spanning Tree Protocol?

Spanning Tree Protocol, commonly referred to as STP, is a network protocol which operates at the Data Link Layer of the OSI model. It was initially standardized in the IEEE 802.1D specification and has since evolved with subsequent versions like Rapid Spanning Tree Protocol (RSTP) and Multiple Spanning Tree Protocol (MSTP).

Spanning Tree Protocol is mostly found in industrial grade fully managed layer 2 switches. The primary objective of STP is to prevent the formation of loops in redundant network topologies. A loop occurs when there are multiple paths between network switches, causing packet collisions, broadcast storms, and severe network degradation. STP reduces this risk by regularly monitoring the network and selectively blocking redundant paths, hence establishing a loop-free logical topology.

How does Spanning Tree Protocol work?

Spanning Tree Protocol is built on bridge protocol data units which are sent back and forth uses an algorithm called the Spanning Tree Algorithm (STA) for ensuring logical loop-free topology. Here's a simplified overview of how STP works:

  • Electing a Root Bridge: In a network, one switch is elected as the Root Bridge, which serves as the reference point for determining the best paths to reach all other switches. The Root Bridge has the lowest Bridge ID, a combination of the Bridge Priority and the MAC address.
  • Calculating the Best Path: Each switch, except the Root Bridge, determines the best path to reach the Root Bridge based on the cost associated with each link. The cost is typically determined by the link speed. Lower-cost paths are preferred.
  • Blocking Redundant Paths: Once the best path to the Root Bridge is determined, STP selectively blocks redundant paths to prevent loops. Blocked paths remain inactive, acting as backups in case of a link failure.
  • Port Roles: STP assigns specific roles to each port on a switch. These roles include Root Port (the port closest to the Root Bridge), Designated Port (the best path to reach a specific segment), and Blocked Port (inactive port to prevent loops).

What are the Types of Spanning Tree Protocol?

Over the years, there is rise in different variations of STP that offer enhanced functionality and improved performance. Following are the types of STP which are commonly used:

  • IEEE 802.1D Spanning Tree Protocol (STP):
    • The IEEE 802.1D STP is the original and most basic version of STP.
    • It uses the Spanning Tree Algorithm (STA) to elect a Root Bridge and calculate the best path to reach the Root Bridge from each switch.
    • However, IEEE 802.1D STP has slower convergence in larger networks.
  • Rapid Spanning Tree Protocol (RSTP):
    • Rapid Spanning Tree Protocol is the improved version of STP
    • It reduces the convergence time in response to network changes, such as link failures or additions.
    • Fast convergence is achieved by introducing new port states and mechanisms like the Alternate Port and Backup Port.
    • RSTP allows seamless integration with existing networks.
  • Multiple Spanning Tree Protocol (MSTP):
    • Multiple Spanning Tree Protocol extends the functionality of STP by to create multiple spanning trees within a network.
    • For designing complex networks, MSTP provide flexibility.
    • MSTP reduces the computational burden on switches and enhances network performance.
  • Per-VLAN Spanning Tree Plus (PVST+):
    • PVST+ is a Cisco proprietary extension of STP that provides a separate spanning tree for each VLAN in the network.
    • It allows more granular control over spanning tree configurations at the VLAN level, enabling optimized forwarding paths for individual VLANs.
    • PVST+ maintains compatibility with IEEE 802.1D STP and allows Cisco network devices to interoperate seamlessly with non-Cisco devices using standard STP.
  • Rapid PVST+:
    • Rapid PVST+ is an enhancement of PVST+.
    • It provides fast convergence times by utilizing the rapid spanning tree techniques for each VLAN.
    • RPVST+ is commonly used in Cisco networks to achieve faster network recovery in VLAN-based environments.

Enhancing Network Security through Spanning Tree Protocol:

Beyond its primary role of ensuring network stability by eliminating loops, Spanning Tree Protocol also contributes to network security in the following ways:

  • Preventing Broadcast Storms: By blocking redundant paths, STP prevents the spread of broadcast storms, which can overwhelm the network and compromise its security and performance.
  • Controlling Unauthorized Network Access: STP allows network administrators to control which ports are active and which are blocked. This capability aids in preventing unauthorized devices from connecting to the Ethernet network by blocking unused or unauthorized ports.
  • Detecting and Responding to Network Changes: STP continuously monitors the network for changes such as link failures, additions, or removal of switches. When a change occurs, STP recalculates the best path and adapts the network accordingly, ensuring uninterrupted connectivity and enhancing the network's resilience to security threats.


Spanning Tree Protocol is a vital network protocol that not only ensures the stability and reliability of a network but also contributes to network security. By preventing loops, controlling network access, and responding to changes, STP creates a secure environment for data transmission and protects against potential network vulnerabilities. Understanding the inner workings of Spanning Tree Protocol enables network administrators to design robust, secure networks that meet the demands of today's interconnected world. Versitron is a leading provider of network connectivity solutions, including a range of layer 2 managed switches, PoE Switches, media converters that are designed to provide reliable and cost-effective connectivity for your networks.

Rich Tull

R.W. Tull is the President of Versitron, a prominent technology company specializing in innovative solutions for data communication and networking. With extensive experience in the industry, R.W. Tull leads the company's strategic vision and oversees its day-to-day operations.  With a deep understanding of data communication technologies and networking systems, R.W. Tull has played a pivotal role in driving Versitron's success.

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