What are Bridge Protocol Data Units

The stable, active STP topology of a switched network is determined by these elements:

• The unique bridge ID (switch priority and MAC address) associated with each
VLAN on each switch
• The STP path cost to the root switch
• The port identifier (port priority and MAC address) associated with each interface
The BPDUs are transmitted in one direction from the root switch, and each switch sends
configuration BPDUs to communicate and to compute the STP topology. Each
configuration BPDU contains this information:
• The unique bridge ID of the switch that the transmitting switch identifies as the
root switch
• The STP path cost to the root
• The bridge ID of the transmitting switch
• Message age
• The identifier of the transmitting interface
• Values for the hello, forward delay, and max-age protocol timers
When a switch transmits a BPDU frame, all switches connected to the LAN on which the
frame is transmitted receive the BPDU. When a switch receives a BPDU, it does not
forward the frame but instead uses the information in the frame to calculate a BPDU, and,
if the topology changes, initiates a BPDU transmission.
A BPDU exchange results in these actions:
• One switch is elected as the root switch.
• The shortest distance to the root switch is calculated for each switch based on the
path cost.
• A designated switch for each LAN segment is selected. The designated switch is
the one closest to the root switch through which frames are forwarded to the root.
• A root port is selected. This port provides the best path from the switch to the root
switch.
• Interfaces included in the spanning-tree instance are selected.
• All interfaces not included in the spanning tree are blocked.