Editing Spanning Tree Protocol (section)

===BPDU types===
There are two types of BPDUs in the original STP specification (802.1D)<ref name="802.1D-2004"/>{{rp|63}} (the Rapid Spanning Tree (RSTP) extension uses a specific RSTP BPDU).

IEEE 802.1D and [[IEEE 802.1aq]] BPDUs have the following format:
{{APHD|start|title=Topology Change Notificatation BPDU}}
{{APHD|0|bits1=16|field1=Protocol ID|value1={{Mono|0x0000}}|bits2=8|field2=Version ID|value2={{Mono|0x00}}|bits3=8|field3=BPDU Type|value3={{Mono|0x80}}}}
{{APHD|end}}
;{{APHD|def|name=Protocol ID|length=16 bits|text=Protocol ID value. This is set to {{Mono|0x0000}} for all BPDUs.}}
;{{APHD|def|name=Version ID|length=8 bits|text=Version ID. Set to {{Mono|0x00}} for TCN BPDUs.}}
;{{APHD|def|name=BPDU Type|length=8 bits|text=BPDU Type is set to {{Mono|0x80}} for TCN BPDUs.}}
Topology change notification (TCN) BPDU, used to announce changes in the network topology, such as port up or port down.
{{APHD|start|title=Configuration BPDU}}
{{APHD|0|bits1=16|field1=Protocol ID|value1={{Mono|0x0000}}|bits2=8|field2=Version ID|bits3=8|field3=BPDU Type}}
{{APHD|4|bits1=8|field1=Flags|bits2=24|border2=bottom|field2={{Nbsp}}}}
{{APHD|8|bits1=32|border1=top-bottom|field1=Root ID}}
{{APHD|12|bits1=8|border1=top|field1={{Nbsp}}|bits2=24|border2=right|field2=Root Path Cost}}
{{APHD|16|bits1=8|border1=left|field1=Root Path Cost (cont.)|bits2=24|border2=bottom|field2={{Nbsp}}}}
{{APHD|20|bits1=32|border1=top-bottom|field1=Bridge ID}}
{{APHD|24|bits1=8|border1=top|field1={{Nbsp}}|bits2=16|field2=Port ID|bits3=8|border3=right|field3=Message Age}}
{{APHD|28|bits1=8|border1=left|field1=Message Age (cont.)|bits2=16|field2=Max Age|bits3=8|border3=right|field3=Hello Time}}
{{APHD|32|bits1=8|border1=left|field1=Hello Time (cont.)|bits2=16|field2=Forward Delay|bits3=8|field3=Version 1 Length}}
{{APHD|36|bits1=16|background1=linen|field1=(Version 3 Length)}}
{{APHD|end}}
;{{APHD|def|name=Protocol ID|length=16 bits|text=Protocol ID value. This is set to {{Mono|0x0000}} for all BPDUs and represents 802.1D.}}
;{{APHD|def|name=Version ID|length=8 bits|text=Version ID. Set to {{Mono|0x00}} for Config and TCN BPDUs; {{Mono|0x02}} indicates a RST BPDU; {{Mono|0x03}} indicates an MST BPDU; and {{Mono|0x04}} indicates an STP BPDU.}}
;{{APHD|def|name=BPDU Type|length=8 bits|text=BPDU Type is set to {{Mono|0x80}} for TCN BPDUs.}}
;{{APHD|def|name=Flags|length=8 bits|text=Flags.}}
:;{{APHD|def|name=Topology Change|length=1 bit|text=0 or 1 for Topology Change.}}
:;{{APHD|def|name=Proposal|length=1 bit|text=0 (unused) or 1 for Proposal in RST/MST/SPT BPDU.}}
:;{{APHD|def|name=Port Role|length=2 bits|text=00 (unused) or{{Break}}01 for Port Role Alternate/Backup in RST/MST/SPT BPDU.{{Break}}10 for Port Role Root in RST/MST/SPT BPDU.{{Break}}11 for Port Role Designated in RST/MST/SPT BPD.}}
:;{{APHD|def|name=Learning|length=1 bit|text=0 (unused) or 1 for Learning in RST/MST/SPT BPDU.}}
:;{{APHD|def|name=Forwarding|length=1 bit|text=0 (unused) or 1 for Forwarding in RST/MST/SPT BPDU.}}
:;{{APHD|def|name=Agreement|length=1 bit|text=0 (unused) or 1 for Agreement in RST/MST/SPT BPDU.}}
:;{{APHD|def|name=TCA|length=1 bit|text=0 or 1 for Topology Change Acknowledgement.}}
;{{APHD|def|name=Root ID|length=64 bits|text=CIST Root ID in MST/SPT BPDU.{{Break}}This field is divided into three parts:}}
:;{{APHD|def|name=Priority|length=4 bits|text=Root Bridge Priority.}}
:;{{APHD|def|name=System ID Extension|length=12 bits|text=Root Bridge System ID Extension.}}
:;{{APHD|def|name=MAC Address|length=48 bits|text=Root Bridge MAC Address.}}
;{{APHD|def|name=Root Path Cost|length=32 bits|text=CIST External Path Cost in MST/SPT BPDU}}
;{{APHD|def|name=Bridge ID|length=64 bits|text=CIST Regional Root ID in MST/SPT BPDU.{{Break}}This field is divided into three parts:}}
:;{{APHD|def|name=Priority|length=4 bits|text=Bridge Priority.}}
:;{{APHD|def|name=System ID Extension|length=12 bits|text=Bridge System ID Extension.}}
:;{{APHD|def|name=MAC Address|length=48 bits|text=Bridge MAC Address.}}
;{{APHD|def|name=Port ID|length=2 bytes|text=Port ID.}}
;{{APHD|def|name=Message Age|length=2 bytes|text=In 1/256 secs.}}
;{{APHD|def|name=Max Age|length=2 bytes|text=In 1/256 secs.}}
;{{APHD|def|name=Hello Time|length=2 bytes|text=In 1/256 secs.}}
;{{APHD|def|name=Forward Delay|length=2 bytes|text=In 1/256 secs.}}
;{{APHD|def|name=Version 1 Length|length=1 byte|text={{Mono|0x00}} no ver 1 protocol info present. RST, MST, SPT BPDU only.}}
;{{APHD|def|name=Version 3 Length|length=2 bytes|text=Version 3 only; MST, SPT BPDU only.}}

Configuration BPDU (CBPDU), used for spanning tree computation and sent by root bridges to provide information to all switches.