H3C Technologies H3C WX3000 Series Unified Switches - Manual (Page 173)

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Table 22-2 Selection of the optimum configuration BPDU
Step Description
1
Upon receiving a configuration BPDU on a port, the device performs the following processing:
z
If the received configuration BPDU has a lower priority than that of the configuration BPDU
generated by the port, the device will discard the received configuration BPDU without doing
any processing on the configuration BPDU of this port.
z
If the received configuration BPDU has a higher priority than that of the configuration BPDU
generated by the port, the device will replace the content of the configuration BPDU generated
by the port with the content of the received configuration BPDU.
2
The device compares the configuration BPDUs of all the ports and chooses the optimum
configuration BPDU.
Principle for configuration BPDU comparison:
z
The configuration BPDU that has the lowest root bridge ID has the highest priority.
z
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.
z
If all configuration BPDUs have the same root path cost, the following configuration BPDU priority
is compared sequentially: designated bridge IDs, designated port IDs, and then the IDs of the ports
on which the configuration BPDUs are received. The device with a higher priority is elected as the
root bridge.
z
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 bridge 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.
z
Selection of the root port and designated ports
The process of selecting the root port and designated ports is as follows:
Table 22-3 Selection of the root port and designated ports
Step Description
1
A non-root-bridge device takes the port on which the optimum configuration BPDU was received
as the root port.
2
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.
z
The root bridge ID is replaced with that of the configuration BPDU of the root port.
z
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.
z
The designated bridge ID is replaced with the ID of this device.
z
The designated port ID is replaced with the ID of this port.