STP protocol frames
STP uses bridge protocol data units (BPDUs), also known as configuration messages, as its protocol frames. This chapter uses BPDUs to represent all types of spanning tree protocol frames.
STP-enabled devices exchange BPDUs to establish a spanning tree. BPDUs contain sufficient information for the devices to complete spanning tree calculation.
STP uses two types of BPDUs, configuration BPDUs and topology change notification (TCN) BPDUs.
Configuration BPDUs
Devices exchange configuration BPDUs to elect the root bridge and determine port roles. Figure 26 shows the configuration BPDU format.
Figure 26: Configuration BPDU format
The payload of a configuration BPDU includes the following fields:
Protocol ID—Fixed at 0x0000, which represents IEEE 802.1d.
Protocol version ID—Spanning tree protocol version ID. The protocol version ID for STP is 0x00.
BPDU type—Type of the BPDU. The value is 0x00 for a configuration BPDU.
Flags—An 8-bit field indicates the purpose of the BPDU. The lowest bit is the Topology Change (TC) flag. The highest bit is the Topology Change Acknowledge (TCA) flag. All other bits are reserved.
Root ID—Root bridge ID formed by the priority and MAC address of the root bridge.
Root path cost—Cost of the path to the root bridge.
Bridge ID—Designated bridge ID formed by the priority and MAC address of the designated bridge.
Port ID—Designated port ID formed by 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 for STP bridges to transit port state.
Devices use the root bridge ID, root path cost, designated bridge ID, designated port ID, message age, max age, hello time, and forward delay for spanning tree calculation.
TCN BPDUs
Devices use TCN BPDUs to announce changes in the network topology. Figure 27 shows the TCN BPDU format.
Figure 27: TCN BPDU format
The payload of a TCN BPDU includes the following fields:
Protocol ID—Fixed at 0x0000, which represents IEEE 802.1d.
Protocol version ID—Spanning tree protocol version ID. The protocol version ID for STP is 0x00.
BPDU type—Type of the BPDU. The value is 0x80 for a TCN BPDU.
A non-root bridge sends TCN BPDUs when one of the following events occurs on the bridge:
A port transits to the forwarding state, and the bridge has a minimum of one designated port.
A port transits from the forwarding or learning state to the blocking state.
The non-root bridge uses TCN BPDUs to notify the root bridge once the network topology changes. The root bridge then sets the TC flag in its configuration BPDU and propagates it to other bridges.