Specifications

8-42 Rapier Switch Software Reference
Software Release 2.7.3
C613-03098-00 REV A
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 and each MSTI. MSTP is compatible with earlier
spanning tree versions; its Bridge Protocol Data Unit (BPDU) formats build on
earlier versions; see “Compatibility with Previous Spanning Tree Protocols” on
page 8-44. Table 8-13 on page 42 shows the standardised 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.
These are shown in Table 8-13.
Table 8-12: MST Configuration identifier
Field Name Description
Format Selector A single octet field whose value of 0 indicates MSTP
operation
Configuration Name A name (up to 32 characters long) that identifies a
particular MST region. The configuration name is defined
using the SET MSTP command.
Revision Level A number representing the region’s revision level. This value
is normally set to 0.
Configuration Digest A 16 octet (HMAC-MD5 based) signature created from the
MST configuration table.
Table 8-13: MST 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 Bit1 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.