H3C S3610-28P Operation Manual page 211

Operation Manual – MSTP
H3C S3610&S5510 Series Ethernet Switches
Note:
Principle for configuration BPDU comparison:
The configuration BPDU that has the lowest root bridge ID has the highest priority.
If all the configuration BPDUs have the same root bridge ID, they will be compared
for their root path costs. If the root path cost in a configuration BPDU plus the path
cost corresponding to this port is S, the configuration BPDU with the smallest S
value has the highest priority.
If all configuration BPDUs have the same root path cost, they will be compared for
their designated bridge IDs, then their designated port IDs, and then the IDs of the
ports on which they are received. The smaller the ID, the higher message priority.
Selection of the root bridge
At network initialization, each STP-compliant device on the network assumes itself to
be the root bridge, with the root bridge ID being its own device ID. By exchanging
configuration BPDUs, the devices compare one another's root bridge ID. The device
with the smallest root bridge ID is elected as the root bridge.
Selection of the root port and designated ports
The process of selecting the root port and designated ports is as follows:
Table 1-3 Selection of the root port and designated ports
Step
1
2
A non-root-ridge device regards the port on which it received the optimum
configuration BPDU as the root port.
Based on the configuration BPDU and the path cost of the root port, the
device calculates a designated port configuration BPDU for each of the rest
ports.
The root bridge ID is replaced with that of the configuration BPDU of the
root port.
The root path cost is replaced with that of the configuration BPDU of the
root port plus the path cost corresponding to the root port.
The designated bridge ID is replaced with the ID of this device.
The designated port ID is replaced with the ID of this port.
Description
1-5
Chapter 1 MSTP Configuration