STP protocol packets

STP uses bridge protocol data units (BPDUs), also known as configuration messages, as its protocol packets. STP-enabled network devices exchange BPDUs to establish a spanning tree.

STP uses the following types of BPDUs:

  • Configuration BPDUs: Used by the network devices to calculate a spanning tree and maintain the spanning tree topology.
  • Topology change notification (TCN) BPDUs: Use to notify network devices of network topology changes.

Configuration BPDUs contain sufficient information for network devices to complete spanning tree calculation. Important fields in a configuration BPDU include the following:

  • Root bridge ID: Priority and MAC address of the root bridge.
  • Root path cost: Cost of the path to the root bridge indicated by the root identifier from the transmitting bridge.
  • Designated bridge ID: Priority and MAC address of the designated bridge.
  • Designated port ID: Priority and global port number of the designated port.
  • Message age: Age of the configuration BPDU while it propagates in the network.
  • Max age: Maximum age of the configuration BPDU stored on the switch.
  • Hello time: Configuration BPDU transmission interval.
  • Forward delay: Delay that STP bridges use to transit port state.