QTECH
Software Configuration Manual
15-188
Table 2
Selection of the optimum configuration BPDU
Step
Description
1
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.
2
The device compares the configuration BPDUs of all the ports and chooses the optimum
configuration BPDU.
&
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 BPDU 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 3 Selection of the root port and designated ports
Step
Description
1
A non-root-ridge device regards the port on which it received the optimum configuration
BPDU 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.
·
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.