Types of Bridge Protocol Data Unit (BPDUs)

In this article, we will understand the types of Bridge Protocol Data Unit (BPDU). BPDU is the messaging mechanism used by switches in the Spanning Tree Protocol (STP) domain to exchange critical network topology information. The Spanning Tree Algorithm uses BPDU transmission to discover and understand the network topology, preventing loops and ensuring optimal path selection.

When devices are connected to switch ports, data transmission doesn't begin immediately. Instead, ports transition through various STP states, and BPDUs are used during this process to determine network topology and enable switches to gather information about each other.

Types of BPDU

There are two main types of BPDU used in Spanning Tree Protocol:

• Configuration BPDU Carries topology information and is used for root bridge election and path cost calculation

• Topology Change Notification (TCN) BPDU Signals topology changes to the root bridge when network changes occur

Configuration BPDU

Configuration BPDUs are generated by the root bridge and propagated throughout the network. They contain essential information including the root bridge ID, path cost to the root, and port priorities. These messages enable switches to maintain a loop-free topology and determine optimal forwarding paths.

Working Process

• Initially, every switch considers itself the root bridge

• Each switch generates and transmits configuration BPDUs at regular intervals (default 2 seconds)

• When a switch receives a configuration BPDU, it compares the received root bridge ID with its own

• If the received root bridge ID is lower (better priority):

  • The switch stops generating configuration BPDUs

  • Updates the BPDU information and forwards it out designated ports

• If the received root bridge ID is higher, the switch continues as root candidate

Key Applications

• Root bridge election through bridge ID comparison

• Loop prevention in network topology

• Optimal path calculation using path costs

• Port role assignment (root, designated, blocked)

Topology Change Notification (TCN) BPDU

TCN BPDUs are generated by non-root switches when they detect topology changes, such as link failures or new connections. These notifications travel upstream toward the root bridge, which then informs the entire network about the change.

Working Process

• A switch detects a topology change (port state transition)

• The switch generates a TCN BPDU and sends it toward the root bridge

• Intermediate switches relay the TCN BPDU upstream to their root ports

• The root bridge acknowledges receipt with a Topology Change Acknowledgment (TCA)

• The root bridge sets the Topology Change flag in subsequent configuration BPDUs

• All switches receive the notification and flush their MAC address tables

BPDU Comparison

Conclusion

BPDUs are fundamental to STP operation, with Configuration BPDUs maintaining loop-free topology and TCN BPDUs ensuring network convergence during changes. Together, they enable switches to build and maintain optimal network topologies while preventing broadcast loops.