STP protocol packets
STP uses bridge protocol data units (BPDUs), also known as configuration messages, as its protocol packets. This chapter uses BPDUs to represent all types of spanning tree protocol packets.
STP-enabled network devices exchange BPDUs to establish a spanning tree. BPDUs contain sufficient information for the network devices to complete spanning tree calculation.
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—Notify network devices of network topology changes.
Configuration BPDUs contain sufficient information for the network devices to complete spanning tree calculation. Important fields in a configuration BPDU include the following:
Root bridge ID—Consisting of the priority and MAC address of the root bridge.
Root path cost—Cost of the path to the root bridge denoted by the root identifier from the transmitting bridge.
Designated bridge ID—Consisting of the priority and MAC address of the designated bridge.
Designated port ID—Consisting of the 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.