Spanning Tree Protocol

In modern Ethernet networks, reliability depends heavily on redundancy.
Network Engineers often connect switches with multiple links so that if one cable fails, another can instantly take over.

Figure 1 – Basic Redundant Network Topology

Why would we have a network loop ?

In a typical network topology you can have multiple paths leading to a destination and having multiple paths can be dangerous. Let me show you with the example below:

How Loops Happen

Imagine a simple network with three interconnected switches (SW1, SW2, SW3) and two devices (PC1 and PC2).

Each switch is linked to the others for redundancy. In this setup:

1. PC1 sends a broadcast request (ARP request to communicate with PC2).

Figure 2 – PC1 Sends an ARP Request (Broadcast)

2. The switches forward the broadcast to all ports excluding the one it arrived.

Figure 3 – SW1 Floods the ARP Broadcast

3. SW2 and SW3 receive the broadcast and again send this broadcast to all of theirs ports, creating a network loop.

Figure 4 – SW2 and SW3 Flood the Broadcast Again (Loop Begins)

Loops can cause severe network disruptions:

Figure 4 – Why Loops Are a Problem

• Broadcast storms: Endless circulation of broadcast frames saturates the network, leaving no bandwidth for legitimate traffic.

• MAC table instability: Switches struggle to learn the correct location of devices, leading to unpredictable behavior.

• Network crashes: Excessive traffic overwhelms the network, potentially causing devices to disconnect.

The Spanning Tree Protocol (STP) solves the problem of loops by blocking redundant paths. In any network segment where multiple paths exist, STP determines the best path and temporarily disables the others.

Figure 6 – How STP Prevents the Loop

By doing so:

• Loops are eliminated: Only one active path is used for communication.

• Backup paths remain available: If the active path fails, a blocked path is automatically activated to maintain connectivity.

• Network performance is stabilized: STP ensures efficient traffic flow without disruptions.

Figure 7 – STP Maintains a Loop-Free Backup Path

Now that we understand the importance of STP in preventing network loops, let’s dive deeper into the foundational concepts in How Spanning Tree Protocol Works.