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Understanding of MSTP - MST BPDUs

Latest reply: May 20, 2021 20:05:34 314 38 28 0 2


The purpose of this post is to present an introduction to the MST BPDUs.


MSTP calculates spanning trees on the basis of Multiple Spanning Tree Bridge Protocol Data Units (MST BPDUs). By transmitting MST BPDUs, spanning tree topologies are computed, network topologies are maintained, and topology changes are conveyed.

Table 1 shows differences between TCN BPDUs, configuration BPDUs defined by STP, RST BPDUs defined by RSTP, and MST BPDUs defined by MSTP.

00x00Configuration BPDU

Table 1 - Differences between BPDUs


Figure 1 shows the MST BPDU format.


Figure 1 - MST BPDU format

The first 36 bytes of an intra-region or inter-region MST BPDU are the same as those of an RST BPDU.

Fields from the 37th byte of an MST BPDU are MSTP-specific. The field MSTI Configuration Messages consists of configuration messages of multiple MSTIs.

Table 2 list the major information carried in an MST BPDU.

Protocol Identifier2Indicates the protocol identifier.
Protocol Version1Indicates the protocol version identifier. 0 indicates STP; 2 indicates RSTP; 3 indicates MSTP.
BPDU Type1

Indicates the BPDU type:

  • 0x00: Configuration BPDU for STP

  • 0x80: TCN BPDU for STP

  • 0x02: RST BPDU or MST BPDU

CIST Flags1Indicates the CIST flags.
CIST Root Identifier8Indicates the CIST root switching device ID.
CIST External Path Cost4Indicates the total path costs from the MST region where the switching device resides to the MST region where the CIST root switching device resides. This value is calculated based on link bandwidth.
CIST Regional Root Identifier8Indicates the ID of the regional root switching device on the CIST, that is, the IST master ID. If the root is in this region, the CIST Regional Root Identifier is the same as the CIST Root Identifier.
CIST Port Identifier2Indicates the ID of the designated port in the IST.
Message Age2Indicates the lifecycle of the BPDU.
Max Age
2Indicates the maximum lifecycle of the BPDU. If the Max Age timer expires, it is considered that the link to the root fails.
Hello Time
2Indicates the Hello timer value. The default value is 2 seconds.
Forward Delay2Indicates the forwarding delay timer. The default value is 15 seconds.
Version 1 Length1Indicates the BPDUv1 length, which has a fixed value of 0.
Version 3 Length2Indicates the BPDUv3 length.
MST Configuration Identifier51Indicates the MST configuration identifier, which has four fields.
CIST Internal Root Path Cost4Indicates the total path costs from the local port to the IST master. This value is calculated based on link bandwidth.
CIST Bridge Identifier8Indicates the ID of the designated switching device on the CIST.
CIST Remaining Hops1Indicates the remaining hops of the BPDU in the CIST.
MSTI Configuration Messages(may be absent)16

Indicates an MSTI configuration message. Each MSTI configuration message occupies 16 bytes. If there are n MSTIs, MSTI configuration messages are of nx16 bytes.

Table 2 - Major information carried in an MST BPDU

Configurable MST BPDU Format

Currently, there are two MST BPDU formats:

  • dot1s: BPDU format defined in IEEE 802.1s.

  • legacy: private BPDU format.

If a port transmits either dot1s or legacy BPDUs by default, the user needs to identify the format of BPDUs sent by the peer, and then runs a command to configure the port to support the peer BPDU format. Once the configuration is incorrect, a loop probably occurs due to incorrect MSTP


By using the stp compliance command, you can configure a port on a Huawei datacom device to automatically adjust the MST BPDU format. With this function, the port automatically adopts the peer BPDU format. The following MST BPDU formats are supported by Huawei datacom devices:

  • auto

  • dot1s

  • legacy

In addition to dot1s and legacy formats, the auto mode allows a port to automatically switch to the BPDU format used by the peer based on BPDUs received from the peer. In this manner, the two ports use the same BPDU format. In auto mode, a port uses the dot1s BPDU format by default, and keeps pace with the peer after receiving BPDUs from the peer.

Configurable Maximum Number of BPDUs Sent by a Port at a Hello Interval...

BPDUs are sent at Hello intervals to maintain the spanning tree. If a switching device does not receive any BPDU during a certain period of time, the spanning tree will be re-calculated.

After a switching device becomes the root, it sends BPDUs at Hello intervals. Non-root switching devices adopt the Hello Time value set for the root.

Huawei datacom devices allow the maximum number of BPDUs sent by a port at a Hello interval to be configured as needed.

The greater the Hello Time value, the more BPDUs sent at a Hello interval. Setting the Hello Time to a proper value limits the number of BPDUs sent by a port at a Hello interval. This helps prevent network topology flapping and avoid excessive use of bandwidth resources by BPDUs.

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MVE Created May 14, 2021 23:28:46

Well Understood
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andersoncf1 Created May 17, 2021 17:23:37 (0) (0)
Thanks dear friend  
MVE Author Created May 15, 2021 00:54:27

Great, thanks to share
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user_3015189 Created May 15, 2021 22:32:04 (0) (0)
lucian2003 Reply user_3015189  Created May 16, 2021 00:52:36 (0) (0)
andersoncf1 Created May 17, 2021 17:23:47 (0) (0)
Thanks dear  
lucian2003 Reply andersoncf1  Created May 18, 2021 00:57:09 (0) (0)
MVE Created May 15, 2021 02:32:46

Easy to understand
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zaheernew Created May 15, 2021 05:41:28 (0) (0)
EL.BODO Created May 15, 2021 16:14:52 (0) (0)
Created May 15, 2021 02:40:34

Very well
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EL.BODO Created May 15, 2021 16:15:11 (0) (0)
Created May 15, 2021 02:48:35

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zaheernew Created May 15, 2021 05:41:41 (0) (0)
Created May 15, 2021 02:58:41

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Created May 15, 2021 03:05:18

Understanding of MSTP - MST BPDUs-3930235-1 Understanding of MSTP - MST BPDUs-3930235-2
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MVE Created May 15, 2021 05:41:15

Understanding of MSTP - MST BPDUs-3930387-1
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Moderator Created May 15, 2021 06:40:28

Looks informative, thanks for sharing
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