H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 54)

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Step Description
3
The device compares the calculated configuration BPDU with the configuration BPDU on the port
whose port role is to be determined:
• If the calculated configuration BPDU is superior, the device considers this port as the
designated port, replaces the configuration BPDU on the port with the calculated configuration
BPDU, and periodically sends out the calculated configuration BPDU.
• If the configuration BPDU on the port is superior, the device blocks this port without updating its
configuration BPDU. The blocked port can receive BPDUs, but cannot send BPDUs or forward
data traffic.
NOTE:
When the network topology is stable, only the root port and designated ports forward user traffic, while
other ports are all in the blocked state to receive BPDUs but not forward BPDUs or user traffic.
Table 9 Selection of the optimum configuration BPDU
Step Actions
1
Upon receiving a configuration BPDU on a port, the device compares the priority of the received
configuration BPDU with that of the configuration BPDU generated by the port, and:
• If the former priority is lower, the device discards the received configuration BPDU and keeps
the configuration BPDU the port generated.
• If the former priority is higher, the device replaces 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:
The following are the principles of configuration BPDU comparison:
• The configuration BPDU with the lowest root bridge ID has the highest priority.
• If configuration BPDUs have the same root bridge ID, their root path costs are compared. For example,
the root path cost in a configuration BPDU plus the path cost of a receiving port is S. The configuration
BPDU with the smallest S value has the highest priority.
• If all configuration BPDUs have the same ports value, their designated bridge IDs, designated port IDs,
and the IDs of the receiving ports are compared in sequence. The configuration BPDU containing a
smaller ID wins out.
A tree-shape topology forms when the root bridge, root ports, and designated ports are selected.
The following describes with an example how the STP algorithm works.