H3C S3100-52P Operation Manual page 249

Operation Manual – MSTP
H3C S3100-52P Ethernet Switch
Table 1-2 Selection of the optimum configuration BPDU
Step
1
2
Note:
Principle for configuration BPDU comparison:
The configuration BPDU that has the lowest root bridge ID has the highest priority.
If all 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, 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
switch with a higher priority is elected as the root bridge.
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.
Selection of the root port and designated ports
The process of selecting the root port and designated ports is as follows:
Upon receiving a configuration BPDU on a port, the device
performs the following processing:
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.
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.
The device compares the configuration BPDUs of all the ports
and chooses the optimum configuration BPDU.
1-5
Chapter 1 MSTP Configuration
Description