Quick Notes - Bridge Protocol Data Units

BPDUs are used for root bridge election and for loop identification. By default, BPDUs are sent out every 2 seconds.

Switches use BPDUs to exchange STP information.

There are two types:

• Configuration BPDUs: Used for calculating the STP

• TCN BPDUs: Used to inform about changes in the network topology

A switch sends out a BPDU on a port. The source address is the MAC address of that port and the destination address is the STP multicast address 01-80-c2-00-00-00.

The BPDU frame consists of the following fields:

• Protocol ID: Identifies the STP
• Version: Identifies the current version of the protocol
• Message type: Identifies the type of BPDU—configuration or TCN BPDU
• Flags: Used in response to a TCN BPDU
• Root bridge ID: Identifies the bridge ID of the root bridge
• Root path cost: Identifies the cost from the transmitting switch to the root
• Sender bridge ID: Identifies the bridge ID of the transmitting switch
• Port ID: Identifies the transmitting port
• Message age: Indicates the age of the current BPDU
• Maximum age: Indicates the timeout value
• Hello time: Identifies the time interval between generation of configuration BPDUs by the root
• Forward delay: Defines the time a switch port must wait in the listening and learning state

  
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