MST BPDUs
MSTP calculates spanning trees using Multiple Spanning Tree Bridge Protocol Data Units (MST BPDUs). MST BPDUs are transmitted to calculate spanning tree topologies, maintain network topologies, and convey topology changes.
Table 10-5 shows differences in the protocol version and type between TCN BPDUs, configuration BPDUs (defined by STP), RST BPDUs (defined by RSTP), and MST BPDUs (defined by MSTP).
Protocol Version |
Type |
Name |
|---|---|---|
0 |
0x00 |
Configuration BPDU |
0 |
0x80 |
TCN BPDU |
2 |
0x02 |
RST BPDU |
3 |
0x02 |
MST BPDU |
MST BPDU Format
Figure 10-9 shows the 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 10-6 lists the major information carried in an MST BPDU.
Field |
Bytes |
Description |
|---|---|---|
Protocol Identifier |
2 |
Indicates the protocol identifier. |
Protocol Version Identifier |
1 |
Indicates the protocol version identifier. 0 indicates STP; 2 indicates RSTP; 3 indicates MSTP. |
BPDU Type |
1 |
Indicates the BPDU type:
|
CIST Flags |
1 |
Indicates the CIST flags. |
CIST Root Identifier |
8 |
Indicates the CIST root switching device ID. |
CIST External Path Cost |
4 |
Indicates the total path cost 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 Identifier |
8 |
Indicates 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 Identifier |
2 |
Indicates the ID of the designated port in the IST. |
Message Age |
2 |
Indicates the lifecycle of the BPDU. |
Max Age |
2 |
Indicates the maximum lifecycle of the BPDU. If the Max Age timer expires, it is considered that the link to the root fails. |
Hello Time |
2 |
Indicates the Hello timer value. The default value is 2 seconds. |
Forward Delay |
2 |
Indicates the forwarding delay timer. The default value is 15 seconds. |
Version 1 Length |
1 |
Indicates the BPDUv1 length, which has a fixed value of 0. |
Version 3 Length |
2 |
Indicates the BPDUv3 length. |
MST Configuration Identifier |
51 |
Indicates the MST configuration identifier, which has four fields. |
CIST Internal Root Path Cost |
4 |
Indicates the total path cost from the local port to the IST master. This value is calculated based on link bandwidth. |
CIST Bridge Identifier |
8 |
Indicates the ID of the designated switching device on the CIST. |
CIST Remaining Hops |
1 |
Indicates 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 occupy n x 16 bytes. |
Configurable MST BPDU Formats
Currently, devices of most vendors support two MST BPDU formats:
dot1s: BPDU format defined in IEEE 802.1s.
legacy: private BPDU format.
There is a possible issue if a port transmits either dot1s or legacy BPDUs by default. That is, the user needs to identify the format of BPDUs sent by the peer, and then run a command to configure the port to support the peer BPDU format. If the configuration is incorrect, a loop will likely occur due to incorrect MSTP calculation.
On Huawei network devices, this issue can be overcome by using the stp compliance command. This command configures a port 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 network devices:
auto
dot1s
legacy
In addition to dot1s and legacy formats, the auto mode allows a port to automatically change 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 changes format according to 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 network 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.