Huawei Quidway S3500 Series Operation Manual page 438

Operation Manual - STP
Quidway S3500 Series Ethernet Switches
To facilitate the descriptions, only the first four parts of the configuration BPDU are
described in the example. They are root ID (expressed as Ethernet switch priority), path
cost to the root, designated switch ID (expressed as Ethernet switch priority) and the
designated port ID (expressed as the port number). As illustrated in the figure above,
the priorities of Switch A, B and C are 0, 1 and 2 and the path costs of their links are 5,
10 and 4 respectively.
1) Initial state
When initialized, each port of the switches will generate the configuration BPDU taking
itself as the root with a root path cost as 0, designated switch IDs as their own switch
IDs and the designated ports as their ports.
Switch A:
Configuration BPDU of AP1: {0, 0, 0, AP1}
Configuration BPDU of AP2: {0, 0, 0, AP2}
Switch B:
Configuration BPDU of BP1: {1, 0, 1, BP1}
Configuration BPDU of BP2: {1, 0, 1, BP2}
Switch C:
Configuration BPDU of CP2: {2, 0, 2, CP2}
Configuration BPDU of CP1: {2, 0, 2, CP1}
2) Select the optimum configuration BPDU
Every switch transmits its configuration BPDU to others. When a port receives a
configuration BPDU with a lower priority than that of its own, it will discard the message
and keep the local BPDU unchanged. When a higher-priority configuration BPDU is
received, the local BPDU is updated. And the optimum configuration BPDU will be
elected through comparing the configuration BPDUs of all the ports.
The comparison rules are:
The configuration BPDU with a smaller root ID has a higher priority
f the root IDs are the same, perform the comparison based on root path costs. The
cost comparison is as follows: the path cost to the root recorded in the
configuration BPDU plus the corresponding path cost of the local port is set as S,
the configuration BPDU with a smaller S has a higher priority.
If the costs of path to the root are also the same, compare in sequence the
designated switch ID, designated port ID and the ID of the port via which the
configuration BPDU was received.
In summary, we assume that the optimum BPDU can be selected through root ID
comparison in the example.
3) Specify the root port, block the redundancy link and update the configuration
BPDU of the designated port.
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Chapter 1 MSTP Region-configuration