14
Figure 8 MSTP BPDU format
The first 13 fields of an MSTP BPDU are the same as an RSTP BPDU. The other six fields are
unique to MSTP.
• Protocol version ID—The value is 0x03 for MSTP.
• BPDU type—The value is 0x02 for RSTP/MSTP BPDUs.
• Root ID—ID of the common root bridge.
• Root path cost—CIST external path cost.
• Bridge ID—ID of the regional root for the IST or an MSTI.
• Port ID—ID of the designated port in the CIST.
• Version3 length—Length of the MSTP-specific fields. Devices use this field for verification
upon receiving an MSTP BPDU.
• MST configuration ID—Includes the format selector, configuration name, revision level, and
configuration digest. The value for format selector is fixed at 0x00. The other parameters are
used to identify the MST region for the originating bridge.
• CIST IRPC—Internal root path cost (IRPC) from the originating bridge to the root of the MST
region.
• CIST bridge ID—ID of the bridge that sends the MSTP BPDU.
• CIST remaining ID—Remaining hop count. This field limits the scale of the MST region. The
regional root sends a BPDU with the remaining hop count set to the maximum value. Each
device that receives the BPDU decrements the hop count by one. When the hop count reaches
zero, the BPDU is discarded. Devices beyond the maximum hops of the MST region cannot
participate in spanning tree calculation. The default remaining hop count is 20.
• MSTI configuration messages—Contains MSTI configuration messages. Each MSTI
configuration message is 16 bytes. This field can contain 0 to 64 MSTI configuration messages.
The number of the MSTI configuration messages is determined by the number of MSTIs in the
MST region.
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