Two important Bridge Protocol Data Unit (BPDU) frames which switches exchange are configuration Bridge Protocol Data Units (BPDUs) and topology change Bridge Protocol Data Units (BPDUs). Configuration Bridge Protocol Data Units (BPDUs) are sent between bridges to establish a network topology. Topology change Bridge Protocol Data Units (BPDUs) are sent after a topology change has been detected to indicate that the spanning-tree algorithm should be initiated.
The format of IEEE 802.1D Bridge Protocol Data Unit (BPDU) is given below.
• Protocol ID (2 bytes): Contains the value 0000 for IEEE 802.1D
• Version ID (1 byte): Contains the value zero.
• BPDU Message Type (1 byte): Configuration or TCN BPDU
• Flags (1 byte): The Topology Change (TC) bit signals a topology change. The Topology Change Acknowledgment (TCA) bit is set to acknowledge receipt of a configuration message.
• Port ID 2 bytes): Identifies the port from which the configuration message was sent.
• Message Age (2 bytes): Specifies the amount of time elapsed since the root sent the configuration message on which the current configuration message is based.
• Maximum Age (2 bytes): Indicates when the current configuration message should be deleted.
• Hello time (2 bytes): Provides the time period between root bridge configuration messages.
• Forward Delay (2 bytes): Provides the length of time that bridges should wait before transitioning to a new state after a topology change.
