Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 485)

Spanning Tree Introduction: STP, RSTP, and MSTP
Software Reference for x310 Series Switches
C613-50046-01 REV A AlliedWare Plus TM Operating System - Version 5.4.4C 18.17
MSTP Bridge Protocol Data Units (BPDUs)
The main function of bridge protocol data units is to enable MSTP to select its root bridges
for the CIST (
“Common and Internal Spanning Tree (CIST)” on page 18.15 ) and each
MSTI. MSTP is compatible with earlier spanning tree versions; its Bridge Protocol Data Unit
(BPDU) formats build on earlier versions (
“Compatibility with Previous Spanning Tree
Protocols” on page 18.15 ).
Table 18-7 shows the standardized format for MSTP BPDU messages. The general format
of the BPDUs comprise a common generic portion—octets 1 to 36—that are based on
those defined in IEEE Standard 802.1D, 1998, followed by components that are specific to
CIST—octets 37 to 102. Components specific to each MSTI are added to this BPDU data
block.
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Table 18-7: MSTP Bridge Protocol Data Units (BPDUs)
Field Name Octets Description
Protocol Identifier 1–2 Protocol being used. The value 0000 0000 0000 0000
identifies the spanning tree algorithm and protocol.
Protocol Version
Identifier 3 Identifies the protocol version used.
BPDU Type 4 Value 0000 0000 specifies a configuration BPDU.
CIST Flags 5 Bit 1 is the topology change flag.
Bit 2 conveys the CIST proposal flag in RST and MST
BPDUs - unused in STP.
Bits 3 & 4 convey the CIST port role in RST, and MST
BPDUs - unused in STP.
Bit 5 conveys the CIST learning flag in RST and MST
BPDUs - unused in STP.
Bit 6 conveys the CIST forwarding flag in RST and MST
BPDUs - unused in STP.
Bit 7 conveys the CIST agreement flag in RST and MST
BPDUs - unused in STP.
Bit 8 conveys the topology change acknowledge flag in
STP configuration BPDUs - unused in RSTP and MSTP
BPDUs.
CIST Root Identifier 6–13 The Bridge identifier of the CIST Root
CIST External Path
Cost 14–17 The path cost between MST regions from the
transmitting bridge to the CIST root.
CIST Regional Root
Identifier 18–25 ID of the current CIST regional root bridge.
CIST Port Identifier 26–27 CIST port identifier of the transmitting bridge port.
Message Age 28–29 Message age timer value.
Max Age 30–31 Timeout value to be used by all bridges in the bridged
network. This value is set by the root. Some
implementations of MSTP may choose not to use this
value.
Hello Time 32–33 Time interval between the generation of configuration
BPDUs by the root bridge.
Forward Delay 34–35 A timeout value used to ensure forward delay timer
consistency when transferring a port to the forwarding
state. It is also used for ageing filtering database
dynamic entries following changes in the active
topology.
Version 1 Length 36 Used to convey the Version 1 length. It is always
transmitted as 0.