General operation of BPDUs

In a working network, each switch sends BPDUs containing the identification of what it believes is the root bridge.

If a switch receives a BPDU indicating that there is a root bridge with an ID lower than the one it currently has, it will update its information and begin forwarding that BPDU instead of its own.

This process ensures that all switches come to an agreement on which is the root bridge. From there, they use the BPDUs to determine the shortest path to the root bridge, and will block any ports that are not on that path to prevent network loops.

It is important to understand how the Spanning Tree protocol uses these packets to maintain a healthy and functional network.

Choosing the Root Bridge

The process begins with choosing the Root Bridge. Each switch sends a Configuration BPDU with its own Bridge ID as the Root Bridge ID. A Bridge ID is composed of a priority value (2 bytes) and the MAC address of the switch (6 bytes). The switch with the lowest Bridge ID becomes the Root Bridge. In case of a tie in priority, the lowest MAC address decides the Root Bridge.

Choice of Root Ports and Designated Ports

Once the Root Bridge is established, the other switches must determine the best path to it. This is done through Root Ports. Each switch, other than the Root Bridge, will select one of its ports as the Root Port, which is the port with the lowest Path Cost to the Root Bridge.

Designated Ports are those that are on the selected path to reach the Root Bridge. Designated Ports are responsible for sending BPDUs down the network hierarchy so that other switches can find their way to the Root Bridge.

Blocking ports and port roles in RSTP

Ports that are not selected as Root Ports or Designated Ports are blocked to prevent loops. In a blocking state, a port will not send or receive data traffic, but can still receive BPDUs.

RSTP, unlike traditional STP, defines additional roles for ports, such as alternate port and backup port. These roles allow for faster reconvergence of the network if topology changes occur.

Changes in topology and BPDUs

When a change in topology occurs, such as a link going down or coming up, the switches involved begin sending TCN BPDUs toward the Root Bridge. When the Root Bridge receives these BPDUs, it sends a Configuration BPDU to all switches to indicate that there has been a change in the topology and that they should update their information.

Closing remarks

It is important to note that the details above describe the standard behavior of the Spanning Tree protocol and how it uses BPDUs. However, there are several versions and variants of the Spanning Tree protocol (such as RSTP, MSTP, PVST, etc.) that can vary in specific details of how they use BPDUs and other aspects of their operation.

In general, BPDUs are essential to maintain the stability and efficiency of Ethernet networks. They allow communication and coordination between switches to avoid loops, select the best path for traffic, and adapt to changes in network topology.