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MSTP provides the following features:
• MSTP divides a switched network into multiple regions, each of which contains multiple
spanning trees that are independent of one another.
• MSTP supports mapping VLANs to spanning tree instances by means of a VLAN-to-instance
mapping table. MSTP can reduce communication overheads and resource usage by mapping
multiple VLANs to one instance.
• MSTP prunes a loop network into a loop-free tree, which avoids proliferation and endless
cycling of frames in a loop network. In addition, it supports load balancing of VLAN data by
providing multiple redundant paths for data forwarding.
• MSTP is compatible with STP and RSTP, and partially compatible with PVST.
MSTP protocol frames
Figure 29 shows the format of an MSTP BPDU.
Figure 29 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.
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