H3C S5560S-EI Series Configuration Manual page 74

Step
3
When the network topology is stable, only the root port and designated ports forward user traffic.
Other ports are all in the blocking state to receive BPDUs but not to forward BPDUs or user
traffic.
Table 6 Selecting the optimum configuration BPDU
Step
1
2
The following are the principles of configuration BPDU comparison:
a. The configuration BPDU with the lowest root bridge ID has the highest priority.
b. 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.
c. If all configuration BPDUs have the same root bridge ID and S value, the following attributes
are compared in sequence:
− Designated bridge IDs.
− Designated port IDs.
− IDs of the receiving ports.
The configuration BPDU that contains a smaller designated bridge ID, designated port ID,
or receiving port ID is selected.
A tree-shape topology forms when the root bridge, root ports, and designated ports are selected.
Example of STP calculation
Figure 20 provides an example showing how the STP algorithm works.
Description
The device compares the calculated configuration BPDU with the configuration BPDU on
the port whose port role will be determined. Then, the device acts depending on the result
of the comparison:
•
If the calculated configuration BPDU is superior, the device performs the following
operations:
Considers this port as the designated port.
Replaces the configuration BPDU on the port with the calculated configuration
BPDU.
Periodically sends 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.
Actions
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.
•
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.
The device compares the configuration BPDUs of all the ports and chooses the optimum
configuration BPDU.
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