How Spanning Tree Protocol Works

In this lesson, we will walk through how the Spanning Tree Protocol works. You will learn how it detects loops, elects the root bridge and activates only the necessary links to ensure a stable network.

Figure 1 - Spanning Tree Protocol Topology

In this network topology with three switches, STP blocks one redundant link to prevent loops. If the active link fails, the previously blocked link becomes active, ensuring uninterrupted communication.

Now, you might wonder : "How does STP determine which paths to block and which to keep active?"

Good question ! It starts by identifying and organizing switches using a unique identifier known as the Bridge ID (BID).

Let’s see how this works.

To manage redundant paths and organize the network, STP assigns a Bridge ID (BID) to each switch. This unique identifier is used to differentiate switches in the spanning tree topology.

What Is a Bridge ID ?

A Bridge ID consists of:

• Priority + VLAN ID: A configurable priority value combined with the VLAN ID. The default priority value is 32,768

• MAC Address: The unique MAC address of the switch (Example : AABB:CCDD:EE01)

Figure 2 – Bridge ID (BID) Structure

A Switch has the following components for its BID:

• Priority + VLAN ID: 32768 + VLAN 1 = 32769.

• MAC Address: AABB:CCDD:EE01.

The BID for SW1, 32769.AABB.CCDD.EE01, uniquely identifies it in the network by combining the priority, VLAN, and MAC address.

As you can see in our topology, every switch has a different BID.

Figure 3 – Bridge IDs in a Spanning Tree Topology

Now that we’ve learned how to identify devices in our Spanning Tree topology using unique identifiers, let’s explore how these devices communicate and exchange information to build and maintain the STP topology.

Bridge Protocol Data Units (BPDUs) are messages exchanged between switches in a Spanning Tree topology. They allow switches to communicate their Bridge ID (BID) and share essential information about the network.

BPDUs are STP messages that switches use to exchange information about the network topology, ensuring all devices work together to prevent loops. These messages are essential for electing the Root Bridge which serves as the central point of the spanning tree.

Figure 4 – BPDU Exchange Between Switches

What Do BPDUs Contain?

BPDUs carry essential information that helps switches determine their roles and relationships within the network: