Mstp Bridge Protocol Data Units (Bpdus) - Allied Telesis SwitchBlade x908 Series Software Reference Manual

Spanning Tree Introduction: STP, RSTP, and MSTP

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
MSTP is compatible with earlier spanning tree versions; its Bridge Protocol Data Unit (BPDU)
formats build on earlier versions
page
Table 18-7
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.
Table 18-7: MSTP Bridge Protocol Data Units (BPDUs)
Field Name
Protocol Identifier
Protocol Version
Identifier
BPDU Type
CIST Flags
CIST Root Identifier
CIST External Path
Cost
CIST Regional Root
Identifier
CIST Port Identifier
Message Age
Max Age
Hello Time
Forward Delay
Version 1 Length
18.16
("Common and Internal Spanning Tree (CIST)" on page
18.14).
shows the standardized format for MSTP BPDU messages. The general format of
Octets
1–2
3
4
5
6–13
14–17
18–25
26–27
28–29
30–31
32–33
34–35
36
Software Reference for SwitchBlade® x908, x900 and x600 Series Switches
TM
AlliedWare Plus Operating System - Software Version 5.3.1
("Compatibility with Previous Spanning Tree Protocols" on
Description
Protocol being used. The value 0000 0000 0000 0000
identifies the spanning tree algorithm and protocol.
Identifies the protocol version used.
Value 0000 0000 specifies a configuration BPDU.
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.
The Bridge identifier of the CIST Root
The path cost between MST regions from the transmitting
bridge to the CIST root.
ID of the current CIST regional root bridge.
CIST port identifier of the transmitting bridge port.
Message age timer value.
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.
Time interval between the generation of configuration
BPDUs by the root bridge.
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.
Used to convey the Version 1 length. It is always
transmitted as 0.
18.14) and each MSTI.
C613-50007-01 REV B